diff --git a/docs/source/internal/utils.rst b/docs/source/internal/utils.rst
index c745917..dc53378 100644
--- a/docs/source/internal/utils.rst
+++ b/docs/source/internal/utils.rst
@@ -66,11 +66,3 @@ The standard library's :func:`fnmatch.fnmatch` is excellent at deciding if a
 filename matches a single pattern. In our use case, however, we typically have
 a list of patterns and want to know if the filename matches any of them. This
 function abstracts that logic away with a little extra logic.
-
-.. autofunction:: flake8.utils.parse_unified_diff
-
-To handle usage of :option:`flake8 --diff`, |Flake8| needs to be able
-to parse the name of the files in the diff as well as the ranges indicated the
-sections that have been changed. This function either accepts the diff as an
-argument or reads the diff from standard-in. It then returns a dictionary with
-filenames as the keys and sets of line numbers as the value.
diff --git a/docs/source/user/options.rst b/docs/source/user/options.rst
index 170c1e7..f9e8b86 100644
--- a/docs/source/user/options.rst
+++ b/docs/source/user/options.rst
@@ -44,8 +44,6 @@ Index of Options
 
 - :option:`flake8 --count`
 
-- :option:`flake8 --diff`
-
 - :option:`flake8 --exclude`
 
 - :option:`flake8 --filename`
@@ -230,27 +228,6 @@ Options and their Descriptions
         count = True
 
 
-.. option:: --diff
-
-    :ref:`Go back to index <top>`
-
-    .. warning::
-
-        Due to hiding potential errors, this option is deprecated and will be
-        removed in a future version.
-
-    Use the unified diff provided on standard in to only check the modified
-    files and report errors included in the diff.
-
-    Command-line example:
-
-    .. prompt:: bash
-
-        git diff -u | flake8 --diff
-
-    This **can not** be specified in config files.
-
-
 .. option:: --exclude=<patterns>
 
     :ref:`Go back to index <top>`
diff --git a/src/flake8/api/legacy.py b/src/flake8/api/legacy.py
index 9635756..ba4339e 100644
--- a/src/flake8/api/legacy.py
+++ b/src/flake8/api/legacy.py
@@ -136,7 +136,6 @@ class StyleGuide:
                     stdin_display_name=self.options.stdin_display_name,
                     filename_patterns=self.options.filename,
                     exclude=self.options.exclude,
-                    is_running_from_diff=self.options.diff,
                 )
             )
             return not paths
diff --git a/src/flake8/checker.py b/src/flake8/checker.py
index d73349f..18b1ff6 100644
--- a/src/flake8/checker.py
+++ b/src/flake8/checker.py
@@ -97,8 +97,6 @@ class Manager:
         #   implementation issues
         # - the user provided stdin and that's not something we can handle
         #   well
-        # - we're processing a diff, which again does not work well with
-        #   multiprocessing and which really shouldn't require multiprocessing
         # - the user provided some awful input
 
         # class state is only preserved when using the `fork` strategy.
@@ -116,13 +114,6 @@ class Manager:
             )
             return 0
 
-        if self.options.diff:
-            LOG.warning(
-                "The --diff option was specified with --jobs but "
-                "they are not compatible. Ignoring --jobs arguments."
-            )
-            return 0
-
         jobs = self.options.jobs
 
         # If the value is "auto", we want to let the multiprocessing library
@@ -169,7 +160,6 @@ class Manager:
                 stdin_display_name=self.options.stdin_display_name,
                 filename_patterns=self.options.filename,
                 exclude=self.exclude,
-                is_running_from_diff=self.options.diff,
             )
         ]
         self.checkers = [c for c in self._all_checkers if c.should_process]
diff --git a/src/flake8/discover_files.py b/src/flake8/discover_files.py
index b8592c8..580d5fd 100644
--- a/src/flake8/discover_files.py
+++ b/src/flake8/discover_files.py
@@ -55,7 +55,6 @@ def expand_paths(
     stdin_display_name: str,
     filename_patterns: Sequence[str],
     exclude: Sequence[str],
-    is_running_from_diff: bool,
 ) -> Generator[str, None, None]:
     """Expand out ``paths`` from commandline to the lintable files."""
     if not paths:
@@ -75,24 +74,16 @@ def expand_paths(
             logger=LOG,
         )
 
-    def is_included(arg: str, fname: str) -> bool:
-        # while running from a diff, the arguments aren't _explicitly_
-        # listed so we still filter them
-        if is_running_from_diff:
-            return utils.fnmatch(fname, filename_patterns)
-        else:
-            return (
-                # always lint `-`
-                fname == "-"
-                # always lint explicitly passed (even if not matching filter)
-                or arg == fname
-                # otherwise, check the file against filtered patterns
-                or utils.fnmatch(fname, filename_patterns)
-            )
-
     return (
         filename
         for path in paths
         for filename in _filenames_from(path, predicate=is_excluded)
-        if is_included(path, filename)
+        if (
+            # always lint `-`
+            filename == "-"
+            # always lint explicitly passed (even if not matching filter)
+            or path == filename
+            # otherwise, check the file against filtered patterns
+            or utils.fnmatch(filename, filename_patterns)
+        )
     )
diff --git a/src/flake8/main/application.py b/src/flake8/main/application.py
index 5178abb..c76fb86 100644
--- a/src/flake8/main/application.py
+++ b/src/flake8/main/application.py
@@ -13,7 +13,6 @@ from flake8 import checker
 from flake8 import defaults
 from flake8 import exceptions
 from flake8 import style_guide
-from flake8 import utils
 from flake8.formatting.base import BaseFormatter
 from flake8.main import debug
 from flake8.main import options
@@ -66,9 +65,6 @@ class Application:
         #: with a non-zero status code
         self.catastrophic_failure = False
 
-        #: The parsed diff information
-        self.parsed_diff: dict[str, set[int]] = {}
-
     def parse_preliminary_options(
         self, argv: Sequence[str]
     ) -> tuple[argparse.Namespace, list[str]]:
@@ -158,13 +154,6 @@ class Application:
             print(json.dumps(info, indent=2, sort_keys=True))
             raise SystemExit(0)
 
-        if self.options.diff:
-            LOG.warning(
-                "the --diff option is deprecated and will be removed in a "
-                "future version."
-            )
-            self.parsed_diff = utils.parse_unified_diff()
-
         for loaded in self.plugins.all_plugins():
             parse_options = getattr(loaded.obj, "parse_options", None)
             if parse_options is None:
@@ -194,9 +183,6 @@ class Application:
             self.options, self.formatter
         )
 
-        if self.options.diff:
-            self.guide.add_diff_ranges(self.parsed_diff)
-
     def make_file_checker_manager(self) -> None:
         """Initialize our FileChecker Manager."""
         assert self.guide is not None
@@ -213,16 +199,9 @@ class Application:
         :class:`~flake8.checker.Manger` instance run the checks it is
         managing.
         """
-        assert self.options is not None
         assert self.file_checker_manager is not None
-        if self.options.diff:
-            files: list[str] | None = sorted(self.parsed_diff)
-            if not files:
-                return
-        else:
-            files = None
 
-        self.file_checker_manager.start(files)
+        self.file_checker_manager.start()
         try:
             self.file_checker_manager.run()
         except exceptions.PluginExecutionFailed as plugin_failed:
diff --git a/src/flake8/main/options.py b/src/flake8/main/options.py
index 86a6cf8..891bae9 100644
--- a/src/flake8/main/options.py
+++ b/src/flake8/main/options.py
@@ -114,7 +114,6 @@ def register_default_options(option_manager: OptionManager) -> None:
     - ``-q``/``--quiet``
     - ``--color``
     - ``--count``
-    - ``--diff``
     - ``--exclude``
     - ``--extend-exclude``
     - ``--filename``
@@ -163,13 +162,6 @@ def register_default_options(option_manager: OptionManager) -> None:
         "all other output.",
     )
 
-    add_option(
-        "--diff",
-        action="store_true",
-        help="(DEPRECATED) Report changes only within line number ranges in "
-        "the unified diff provided on standard in by the user.",
-    )
-
     add_option(
         "--exclude",
         metavar="patterns",
diff --git a/src/flake8/style_guide.py b/src/flake8/style_guide.py
index 2fee0f3..94698f8 100644
--- a/src/flake8/style_guide.py
+++ b/src/flake8/style_guide.py
@@ -298,18 +298,6 @@ class StyleGuideManager:
             code, filename, line_number, column_number, text, physical_line
         )
 
-    def add_diff_ranges(self, diffinfo: dict[str, set[int]]) -> None:
-        """Update the StyleGuides to filter out information not in the diff.
-
-        This provides information to the underlying StyleGuides so that only
-        the errors in the line number ranges are reported.
-
-        :param diffinfo:
-            Dictionary mapping filenames to sets of line number ranges.
-        """
-        for guide in self.style_guides:
-            guide.add_diff_ranges(diffinfo)
-
 
 class StyleGuide:
     """Manage a Flake8 user's style guide."""
@@ -333,7 +321,6 @@ class StyleGuide:
         self.filename = filename
         if self.filename:
             self.filename = utils.normalize_path(self.filename)
-        self._parsed_diff: dict[str, set[int]] = {}
 
     def __repr__(self) -> str:
         """Make it easier to debug which StyleGuide we're using."""
@@ -440,20 +427,8 @@ class StyleGuide:
             self.should_report_error(error.code) is Decision.Selected
         )
         is_not_inline_ignored = error.is_inline_ignored(disable_noqa) is False
-        is_included_in_diff = error.is_in(self._parsed_diff)
-        if error_is_selected and is_not_inline_ignored and is_included_in_diff:
+        if error_is_selected and is_not_inline_ignored:
             self.formatter.handle(error)
             self.stats.record(error)
             return 1
         return 0
-
-    def add_diff_ranges(self, diffinfo: dict[str, set[int]]) -> None:
-        """Update the StyleGuide to filter out information not in the diff.
-
-        This provides information to the StyleGuide so that only the errors
-        in the line number ranges are reported.
-
-        :param diffinfo:
-            Dictionary mapping filenames to sets of line number ranges.
-        """
-        self._parsed_diff = diffinfo
diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index 60555a9..afc3896 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -1,7 +1,6 @@
 """Utility methods for flake8."""
 from __future__ import annotations
 
-import collections
 import fnmatch as _fnmatch
 import functools
 import io
@@ -18,7 +17,6 @@ from typing import Sequence
 
 from flake8 import exceptions
 
-DIFF_HUNK_REGEXP = re.compile(r"^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@.*$")
 COMMA_SEPARATED_LIST_RE = re.compile(r"[,\s]")
 LOCAL_PLUGIN_LIST_RE = re.compile(r"[,\t\n\r\f\v]")
 NORMALIZE_PACKAGE_NAME_RE = re.compile(r"[-_.]+")
@@ -202,71 +200,6 @@ def stdin_get_lines() -> list[str]:
     return list(io.StringIO(stdin_get_value()))
 
 
-def parse_unified_diff(diff: str | None = None) -> dict[str, set[int]]:
-    """Parse the unified diff passed on stdin.
-
-    :returns:
-        dictionary mapping file names to sets of line numbers
-    """
-    # Allow us to not have to patch out stdin_get_value
-    if diff is None:
-        diff = stdin_get_value()
-
-    number_of_rows = None
-    current_path = None
-    parsed_paths: dict[str, set[int]] = collections.defaultdict(set)
-    for line in diff.splitlines():
-        if number_of_rows:
-            if not line or line[0] != "-":
-                number_of_rows -= 1
-            # We're in the part of the diff that has lines starting with +, -,
-            # and ' ' to show context and the changes made. We skip these
-            # because the information we care about is the filename and the
-            # range within it.
-            # When number_of_rows reaches 0, we will once again start
-            # searching for filenames and ranges.
-            continue
-
-        # NOTE(sigmavirus24): Diffs that we support look roughly like:
-        #    diff a/file.py b/file.py
-        #    ...
-        #    --- a/file.py
-        #    +++ b/file.py
-        # Below we're looking for that last line. Every diff tool that
-        # gives us this output may have additional information after
-        # ``b/file.py`` which it will separate with a \t, e.g.,
-        #    +++ b/file.py\t100644
-        # Which is an example that has the new file permissions/mode.
-        # In this case we only care about the file name.
-        if line[:3] == "+++":
-            current_path = line[4:].split("\t", 1)[0]
-            # NOTE(sigmavirus24): This check is for diff output from git.
-            if current_path[:2] == "b/":
-                current_path = current_path[2:]
-            # We don't need to do anything else. We have set up our local
-            # ``current_path`` variable. We can skip the rest of this loop.
-            # The next line we will see will give us the hung information
-            # which is in the next section of logic.
-            continue
-
-        hunk_match = DIFF_HUNK_REGEXP.match(line)
-        # NOTE(sigmavirus24): pep8/pycodestyle check for:
-        #    line[:3] == '@@ '
-        # But the DIFF_HUNK_REGEXP enforces that the line start with that
-        # So we can more simply check for a match instead of slicing and
-        # comparing.
-        if hunk_match:
-            (row, number_of_rows) = (
-                1 if not group else int(group) for group in hunk_match.groups()
-            )
-            assert current_path is not None
-            parsed_paths[current_path].update(range(row, row + number_of_rows))
-
-    # We have now parsed our diff into a dictionary that looks like:
-    #    {'file.py': set(range(10, 16), range(18, 20)), ...}
-    return parsed_paths
-
-
 def is_using_stdin(paths: list[str]) -> bool:
     """Determine if we're going to read from stdin.
 
diff --git a/src/flake8/violation.py b/src/flake8/violation.py
index 45834b2..96161d4 100644
--- a/src/flake8/violation.py
+++ b/src/flake8/violation.py
@@ -67,36 +67,3 @@ class Violation(NamedTuple):
             "%r is not ignored inline with ``# noqa: %s``", self, codes_str
         )
         return False
-
-    def is_in(self, diff: dict[str, set[int]]) -> bool:
-        """Determine if the violation is included in a diff's line ranges.
-
-        This function relies on the parsed data added via
-        :meth:`~StyleGuide.add_diff_ranges`. If that has not been called and
-        we are not evaluating files in a diff, then this will always return
-        True. If there are diff ranges, then this will return True if the
-        line number in the error falls inside one of the ranges for the file
-        (and assuming the file is part of the diff data). If there are diff
-        ranges, this will return False if the file is not part of the diff
-        data or the line number of the error is not in any of the ranges of
-        the diff.
-
-        :returns:
-            True if there is no diff or if the error is in the diff's line
-            number ranges. False if the error's line number falls outside
-            the diff's line number ranges.
-        """
-        if not diff:
-            return True
-
-        # NOTE(sigmavirus24): The parsed diff will be a defaultdict with
-        # a set as the default value (if we have received it from
-        # flake8.utils.parse_unified_diff). In that case ranges below
-        # could be an empty set (which is False-y) or if someone else
-        # is using this API, it could be None. If we could guarantee one
-        # or the other, we would check for it more explicitly.
-        line_numbers = diff.get(self.filename)
-        if not line_numbers:
-            return False
-
-        return self.line_number in line_numbers
diff --git a/tests/fixtures/diffs/multi_file_diff b/tests/fixtures/diffs/multi_file_diff
deleted file mode 100644
index de86209..0000000
--- a/tests/fixtures/diffs/multi_file_diff
+++ /dev/null
@@ -1,130 +0,0 @@
-diff --git a/flake8/utils.py b/flake8/utils.py
-index f6ce384..7cd12b0 100644
---- a/flake8/utils.py
-+++ b/flake8/utils.py
-@@ -75,8 +75,8 @@ def stdin_get_value():
-     return cached_value.getvalue()
- 
- 
--def parse_unified_diff():
--    # type: () -> List[str]
-+def parse_unified_diff(diff=None):
-+    # type: (str) -> List[str]
-     """Parse the unified diff passed on stdin.
- 
-     :returns:
-@@ -84,7 +84,10 @@ def parse_unified_diff():
-     :rtype:
-         dict
-     """
--    diff = stdin_get_value()
-+    # Allow us to not have to patch out stdin_get_value
-+    if diff is None:
-+        diff = stdin_get_value()
-+
-     number_of_rows = None
-     current_path = None
-     parsed_paths = collections.defaultdict(set)
-diff --git a/tests/fixtures/diffs/single_file_diff b/tests/fixtures/diffs/single_file_diff
-new file mode 100644
-index 0000000..77ca534
---- /dev/null
-+++ b/tests/fixtures/diffs/single_file_diff
-@@ -0,0 +1,27 @@
-+diff --git a/flake8/utils.py b/flake8/utils.py
-+index f6ce384..7cd12b0 100644
-+--- a/flake8/utils.py
-++++ b/flake8/utils.py
-+@@ -75,8 +75,8 @@ def stdin_get_value():
-+     return cached_value.getvalue()
-+
-+
-+-def parse_unified_diff():
-+-    # type: () -> List[str]
-++def parse_unified_diff(diff=None):
-++    # type: (str) -> List[str]
-+     """Parse the unified diff passed on stdin.
-+
-+     :returns:
-+@@ -84,7 +84,10 @@ def parse_unified_diff():
-+     :rtype:
-+         dict
-+     """
-+-    diff = stdin_get_value()
-++    # Allow us to not have to patch out stdin_get_value
-++    if diff is None:
-++        diff = stdin_get_value()
-++
-+     number_of_rows = None
-+     current_path = None
-+     parsed_paths = collections.defaultdict(set)
-diff --git a/tests/fixtures/diffs/two_file_diff b/tests/fixtures/diffs/two_file_diff
-new file mode 100644
-index 0000000..5bd35cd
---- /dev/null
-+++ b/tests/fixtures/diffs/two_file_diff
-@@ -0,0 +1,45 @@
-+diff --git a/flake8/utils.py b/flake8/utils.py
-+index f6ce384..7cd12b0 100644
-+--- a/flake8/utils.py
-++++ b/flake8/utils.py
-+@@ -75,8 +75,8 @@ def stdin_get_value():
-+     return cached_value.getvalue()
-+
-+
-+-def parse_unified_diff():
-+-    # type: () -> List[str]
-++def parse_unified_diff(diff=None):
-++    # type: (str) -> List[str]
-+     """Parse the unified diff passed on stdin.
-+
-+     :returns:
-+@@ -84,7 +84,10 @@ def parse_unified_diff():
-+     :rtype:
-+         dict
-+     """
-+-    diff = stdin_get_value()
-++    # Allow us to not have to patch out stdin_get_value
-++    if diff is None:
-++        diff = stdin_get_value()
-++
-+     number_of_rows = None
-+     current_path = None
-+     parsed_paths = collections.defaultdict(set)
-+diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
-+index d69d939..21482ce 100644
-+--- a/tests/unit/test_utils.py
-++++ b/tests/unit/test_utils.py
-+@@ -115,3 +115,13 @@ def test_parameters_for_function_plugin():
-+     plugin = plugin_manager.Plugin('plugin-name', object())
-+     plugin._plugin = fake_plugin
-+     assert utils.parameters_for(plugin) == ['physical_line', 'self', 'tree']
-++
-++
-++def read_diff_file(filename):
-++    """Read the diff file in its entirety."""
-++    with open(filename, 'r') as fd:
-++        content = fd.read()
-++    return content
-++
-++
-++SINGLE_FILE_DIFF = read_diff_file('tests/fixtures/diffs/single_file_diff')
-diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
-index d69d939..1461369 100644
---- a/tests/unit/test_utils.py
-+++ b/tests/unit/test_utils.py
-@@ -115,3 +115,14 @@ def test_parameters_for_function_plugin():
-     plugin = plugin_manager.Plugin('plugin-name', object())
-     plugin._plugin = fake_plugin
-     assert utils.parameters_for(plugin) == ['physical_line', 'self', 'tree']
-+
-+
-+def read_diff_file(filename):
-+    """Read the diff file in its entirety."""
-+    with open(filename, 'r') as fd:
-+        content = fd.read()
-+    return content
-+
-+
-+SINGLE_FILE_DIFF = read_diff_file('tests/fixtures/diffs/single_file_diff')
-+TWO_FILE_DIFF = read_diff_file('tests/fixtures/diffs/two_file_diff')
diff --git a/tests/fixtures/diffs/single_file_diff b/tests/fixtures/diffs/single_file_diff
deleted file mode 100644
index 77ca534..0000000
--- a/tests/fixtures/diffs/single_file_diff
+++ /dev/null
@@ -1,27 +0,0 @@
-diff --git a/flake8/utils.py b/flake8/utils.py
-index f6ce384..7cd12b0 100644
---- a/flake8/utils.py
-+++ b/flake8/utils.py
-@@ -75,8 +75,8 @@ def stdin_get_value():
-     return cached_value.getvalue()
-
-
--def parse_unified_diff():
--    # type: () -> List[str]
-+def parse_unified_diff(diff=None):
-+    # type: (str) -> List[str]
-     """Parse the unified diff passed on stdin.
-
-     :returns:
-@@ -84,7 +84,10 @@ def parse_unified_diff():
-     :rtype:
-         dict
-     """
--    diff = stdin_get_value()
-+    # Allow us to not have to patch out stdin_get_value
-+    if diff is None:
-+        diff = stdin_get_value()
-+
-     number_of_rows = None
-     current_path = None
-     parsed_paths = collections.defaultdict(set)
diff --git a/tests/fixtures/diffs/two_file_diff b/tests/fixtures/diffs/two_file_diff
deleted file mode 100644
index 5bd35cd..0000000
--- a/tests/fixtures/diffs/two_file_diff
+++ /dev/null
@@ -1,45 +0,0 @@
-diff --git a/flake8/utils.py b/flake8/utils.py
-index f6ce384..7cd12b0 100644
---- a/flake8/utils.py
-+++ b/flake8/utils.py
-@@ -75,8 +75,8 @@ def stdin_get_value():
-     return cached_value.getvalue()
- 
- 
--def parse_unified_diff():
--    # type: () -> List[str]
-+def parse_unified_diff(diff=None):
-+    # type: (str) -> List[str]
-     """Parse the unified diff passed on stdin.
- 
-     :returns:
-@@ -84,7 +84,10 @@ def parse_unified_diff():
-     :rtype:
-         dict
-     """
--    diff = stdin_get_value()
-+    # Allow us to not have to patch out stdin_get_value
-+    if diff is None:
-+        diff = stdin_get_value()
-+
-     number_of_rows = None
-     current_path = None
-     parsed_paths = collections.defaultdict(set)
-diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
-index d69d939..21482ce 100644
---- a/tests/unit/test_utils.py
-+++ b/tests/unit/test_utils.py
-@@ -115,3 +115,13 @@ def test_parameters_for_function_plugin():
-     plugin = plugin_manager.Plugin('plugin-name', object())
-     plugin._plugin = fake_plugin
-     assert utils.parameters_for(plugin) == ['physical_line', 'self', 'tree']
-+
-+
-+def read_diff_file(filename):
-+    """Read the diff file in its entirety."""
-+    with open(filename, 'r') as fd:
-+        content = fd.read()
-+    return content
-+
-+
-+SINGLE_FILE_DIFF = read_diff_file('tests/fixtures/diffs/single_file_diff')
diff --git a/tests/integration/test_main.py b/tests/integration/test_main.py
index dfa0e0b..db60f6b 100644
--- a/tests/integration/test_main.py
+++ b/tests/integration/test_main.py
@@ -13,42 +13,6 @@ from flake8.main import cli
 from flake8.options import config
 
 
-def test_diff_option(tmpdir, capsys):
-    """Ensure that `flake8 --diff` works."""
-    t_py_contents = """\
-import os
-import sys  # unused but not part of diff
-
-print('(to avoid trailing whitespace in test)')
-print('(to avoid trailing whitespace in test)')
-print(os.path.join('foo', 'bar'))
-
-y  # part of the diff and an error
-"""
-
-    diff = """\
-diff --git a/t.py b/t.py
-index d64ac39..7d943de 100644
---- a/t.py
-+++ b/t.py
-@@ -4,3 +4,5 @@ import sys  # unused but not part of diff
- print('(to avoid trailing whitespace in test)')
- print('(to avoid trailing whitespace in test)')
- print(os.path.join('foo', 'bar'))
-+
-+y  # part of the diff and an error
-"""
-
-    with mock.patch.object(utils, "stdin_get_value", return_value=diff):
-        with tmpdir.as_cwd():
-            tmpdir.join("t.py").write(t_py_contents)
-            assert cli.main(["--diff"]) == 1
-
-    out, err = capsys.readouterr()
-    assert out == "t.py:8:1: F821 undefined name 'y'\n"
-    assert err == ""
-
-
 def test_form_feed_line_split(tmpdir, capsys):
     """Test that form feed is treated the same for stdin."""
     src = "x=1\n\f\ny=1\n"
diff --git a/tests/unit/test_checker_manager.py b/tests/unit/test_checker_manager.py
index 32bc9a4..b75bf6d 100644
--- a/tests/unit/test_checker_manager.py
+++ b/tests/unit/test_checker_manager.py
@@ -14,12 +14,7 @@ from flake8.plugins import finder
 
 def style_guide_mock():
     """Create a mock StyleGuide object."""
-    return mock.MagicMock(
-        **{
-            "options.diff": False,
-            "options.jobs": JobsArgument("4"),
-        }
-    )
+    return mock.MagicMock(**{"options.jobs": JobsArgument("4")})
 
 
 def _parallel_checker_manager():
diff --git a/tests/unit/test_discover_files.py b/tests/unit/test_discover_files.py
index edf047d..ca945c2 100644
--- a/tests/unit/test_discover_files.py
+++ b/tests/unit/test_discover_files.py
@@ -125,7 +125,6 @@ def _expand_paths(
     stdin_display_name="stdin",
     filename_patterns=("*.py",),
     exclude=(),
-    is_running_from_diff=False,
 ):
     return set(
         expand_paths(
@@ -133,7 +132,6 @@ def _expand_paths(
             stdin_display_name=stdin_display_name,
             filename_patterns=filename_patterns,
             exclude=exclude,
-            is_running_from_diff=is_running_from_diff,
         )
     )
 
@@ -166,11 +164,3 @@ def test_alternate_stdin_name_is_filtered():
 def test_filename_included_even_if_not_matching_include(tmp_path):
     some_file = str(tmp_path.joinpath("some/file"))
     assert _expand_paths(paths=(some_file,)) == {some_file}
-
-
-def test_diff_filenames_filtered_by_patterns(tmp_path):
-    f1 = str(tmp_path.joinpath("f1"))
-    f2 = str(tmp_path.joinpath("f2.py"))
-
-    ret = _expand_paths(paths=(f1, f2), is_running_from_diff=True)
-    assert ret == {f2}
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index 98a5a5d..82eef63 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -183,44 +183,6 @@ def test_fnmatch(filename, patterns, expected):
     assert utils.fnmatch(filename, patterns) is expected
 
 
-def read_diff_file(filename):
-    """Read the diff file in its entirety."""
-    with open(filename) as fd:
-        content = fd.read()
-    return content
-
-
-SINGLE_FILE_DIFF = read_diff_file("tests/fixtures/diffs/single_file_diff")
-SINGLE_FILE_INFO = {
-    "flake8/utils.py": set(range(75, 83)).union(set(range(84, 94))),
-}
-TWO_FILE_DIFF = read_diff_file("tests/fixtures/diffs/two_file_diff")
-TWO_FILE_INFO = {
-    "flake8/utils.py": set(range(75, 83)).union(set(range(84, 94))),
-    "tests/unit/test_utils.py": set(range(115, 128)),
-}
-MULTI_FILE_DIFF = read_diff_file("tests/fixtures/diffs/multi_file_diff")
-MULTI_FILE_INFO = {
-    "flake8/utils.py": set(range(75, 83)).union(set(range(84, 94))),
-    "tests/unit/test_utils.py": set(range(115, 129)),
-    "tests/fixtures/diffs/single_file_diff": set(range(1, 28)),
-    "tests/fixtures/diffs/two_file_diff": set(range(1, 46)),
-}
-
-
-@pytest.mark.parametrize(
-    "diff, parsed_diff",
-    [
-        (SINGLE_FILE_DIFF, SINGLE_FILE_INFO),
-        (TWO_FILE_DIFF, TWO_FILE_INFO),
-        (MULTI_FILE_DIFF, MULTI_FILE_INFO),
-    ],
-)
-def test_parse_unified_diff(diff, parsed_diff):
-    """Verify that what we parse from a diff matches expectations."""
-    assert utils.parse_unified_diff(diff) == parsed_diff
-
-
 def test_stdin_get_value_crlf():
     """Ensure that stdin is normalized from crlf to lf."""
     stdin = io.TextIOWrapper(io.BytesIO(b"1\r\n2\r\n"), "UTF-8")
diff --git a/tests/unit/test_violation.py b/tests/unit/test_violation.py
index a4a43da..1b4852b 100644
--- a/tests/unit/test_violation.py
+++ b/tests/unit/test_violation.py
@@ -51,22 +51,3 @@ def test_disable_is_inline_ignored():
         assert error.is_inline_ignored(True) is False
 
     assert getline.called is False
-
-
-@pytest.mark.parametrize(
-    "violation_file,violation_line,diff,expected",
-    [
-        ("file.py", 10, {}, True),
-        ("file.py", 1, {"file.py": range(1, 2)}, True),
-        ("file.py", 10, {"file.py": range(1, 2)}, False),
-        ("file.py", 1, {"other.py": range(1, 2)}, False),
-        ("file.py", 10, {"other.py": range(1, 2)}, False),
-    ],
-)
-def test_violation_is_in_diff(violation_file, violation_line, diff, expected):
-    """Verify that we find violations within a diff."""
-    violation = Violation(
-        "E001", violation_file, violation_line, 1, "warning", "line"
-    )
-
-    assert violation.is_in(diff) is expected