API: deprecate unused helper in patch._Styles

Author: tacaswell

doc/api/next_api_changes/deprecations/28670-TAC.rst

@@ -0,0 +1,6 @@
+Drerecated ``matplotlib.patches._Styles`` and subclasses
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This class method is never used internally.  Due to the internal check in the
+method it only accepts subclasses of a private baseclass embedded in the host
+class which makes it unlikely that it has been used externally.

lib/matplotlib/patches.py

@@ -2347,6 +2347,11 @@ def pprint_styles(cls):
         return textwrap.indent(rst_table, prefix=' ' * 4)
 
     @classmethod
+    @_api.deprecated(
+        '3.10.0',
+        message="This method is never used internally.",
+        alternative="No replacement.  Please open an issue if you use this."
+    )
     def register(cls, name, style):
         """Register a new style."""
         if not issubclass(style, cls._Base):