matplotlib
diff --git a/‎doc/api/next_api_changes/deprecations.rst
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+5Lines changed: 5 additions & 0 deletions b/‎doc/api/next_api_changes/deprecations.rst
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diff --git a/‎lib/matplotlib/artist.py
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+6-1Lines changed: 6 additions & 1 deletion b/‎lib/matplotlib/artist.py
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+6-1Lines changed: 6 additions & 1 deletion
@@ -415,3 +415,8 @@ parent figure, though they can be later removed with ``ax.remove()``.
 
 ``.BboxBase.inverse_transformed`` is deprecated (call `.BboxBase.transformed`
 on the `~.Transform.inverted()` transform instead).
+
+Case-insensitive properties
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Support for upper or mixed-case property names in `.Artist.set` and
+`.Artist.update` is deprecated.  Pass properties in lowercase.
@@ -980,7 +980,12 @@ def update(self, props):
         ret = []
         with cbook._setattr_cm(self, eventson=False):
             for k, v in props.items():
-                k = k.lower()
+                if k != k.lower():
+                    cbook.warn_deprecated(
+                        "3.3", message="Case-insensitive properties were "
+                        "deprecated in %(since)s and support will be removed "
+                        "%(removal)s")
+                    k = k.lower()
                 # White list attributes we want to be able to update through
                 # art.update, art.set, setp.
                 if k == "axes":