diff --git a/lib/matplotlib/_fontconfig_pattern.py b/lib/matplotlib/_fontconfig_pattern.py
index d3933b9f396d..5d23a65b56ad 100644
--- a/lib/matplotlib/_fontconfig_pattern.py
+++ b/lib/matplotlib/_fontconfig_pattern.py
@@ -15,8 +15,6 @@
 from pyparsing import (
     Group, Optional, ParseException, Regex, StringEnd, Suppress, ZeroOrMore)
 
-from matplotlib import _api
-
 
 _family_punc = r'\\\-:,'
 _family_unescape = partial(re.compile(r'\\(?=[%s])' % _family_punc).sub, '')
@@ -98,11 +96,7 @@ def parse_fontconfig_pattern(pattern):
     for prop in parse.get("properties", []):
         if len(prop) == 1:
             if prop[0] not in _CONSTANTS:
-                _api.warn_deprecated(
-                    "3.7", message=f"Support for unknown constants "
-                    f"({prop[0]!r}) is deprecated since %(since)s and "
-                    f"will be removed %(removal)s.")
-                continue
+                raise ValueError(f"Unknown constants ({prop[0]!r}) are not supported.")
             prop = _CONSTANTS[prop[0]]
         k, *v = prop
         props.setdefault(k, []).extend(map(_value_unescape, v))
diff --git a/lib/matplotlib/tests/test_fontconfig_pattern.py b/lib/matplotlib/tests/test_fontconfig_pattern.py
index 792a8ed517c2..2e11a182edba 100644
--- a/lib/matplotlib/tests/test_fontconfig_pattern.py
+++ b/lib/matplotlib/tests/test_fontconfig_pattern.py
@@ -1,7 +1,7 @@
-import pytest
 
 from matplotlib.font_manager import FontProperties
 
+import pytest
 
 # Attributes on FontProperties object to check for consistency
 keys = [
@@ -73,5 +73,5 @@ def test_fontconfig_str():
 
 
 def test_fontconfig_unknown_constant():
-    with pytest.warns(DeprecationWarning):
+    with pytest.raises(ValueError):
         FontProperties(":unknown")