diff --git a/lib/matplotlib/_fontconfig_pattern.py b/lib/matplotlib/_fontconfig_pattern.py
index d3933b9f396d..2a2eecc8038c 100644
--- a/lib/matplotlib/_fontconfig_pattern.py
+++ b/lib/matplotlib/_fontconfig_pattern.py
@@ -98,11 +98,7 @@ 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("Support for unknown constants is not supported.")
             prop = _CONSTANTS[prop[0]]
         k, *v = prop
         props.setdefault(k, []).extend(map(_value_unescape, v))