diff --git a/doc/api/next_api_changes/deprecations/30329-ES.rst b/doc/api/next_api_changes/deprecations/30329-ES.rst
new file mode 100644
index 000000000000..8d5060c4821b
--- /dev/null
+++ b/doc/api/next_api_changes/deprecations/30329-ES.rst
@@ -0,0 +1,4 @@
+``font_manager.is_opentype_cff_font`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There is no replacement.
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
index ab6b495631de..79e088b85998 100644
--- a/lib/matplotlib/font_manager.py
+++ b/lib/matplotlib/font_manager.py
@@ -1539,7 +1539,7 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
         return _cached_realpath(result)
 
 
-@lru_cache
+@_api.deprecated("3.11")
 def is_opentype_cff_font(filename):
     """
     Return whether the given font is a Postscript Compact Font Format Font
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
index 24421b8e30b3..b15647644e04 100644
--- a/lib/matplotlib/tests/test_font_manager.py
+++ b/lib/matplotlib/tests/test_font_manager.py
@@ -67,12 +67,14 @@ def test_json_serialization(tmp_path):
 def test_otf():
     fname = '/usr/share/fonts/opentype/freefont/FreeMono.otf'
     if Path(fname).exists():
-        assert is_opentype_cff_font(fname)
+        with pytest.warns(mpl.MatplotlibDeprecationWarning):
+            assert is_opentype_cff_font(fname)
     for f in fontManager.ttflist:
         if 'otf' in f.fname:
             with open(f.fname, 'rb') as fd:
                 res = fd.read(4) == b'OTTO'
-            assert res == is_opentype_cff_font(f.fname)
+            with pytest.warns(mpl.MatplotlibDeprecationWarning):
+                assert res == is_opentype_cff_font(f.fname)
 
 
 @pytest.mark.skipif(sys.platform == "win32" or not has_fclist,