deprecate the --diff option

docs/source/user/options.rst

@@ -238,6 +238,11 @@ Options and their Descriptions
 
     :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.
 

src/flake8/main/application.py

@@ -191,6 +191,10 @@ class Application:
 
         self.running_against_diff = self.options.diff
         if self.running_against_diff:
+            LOG.warning(
+                "the --diff option is deprecated and will be removed in a "
+                "future version."
+            )
             self.parsed_diff = utils.parse_unified_diff()
             if not self.parsed_diff:
                 self.exit()

src/flake8/main/options.py

@@ -146,8 +146,8 @@ def register_default_options(option_manager):
     add_option(
         "--diff",
         action="store_true",
-        help="Report changes only within line number ranges in the unified "
+        help="(DEPRECATED) Report changes only within line number ranges in "
-        "diff provided on standard in by the user.",
+        "the unified diff provided on standard in by the user.",
     )
 
     add_option(