Add compatibility notice after #78266

misc/extension_api_validation/4.0-stable.expected

@@ -433,3 +433,13 @@ GH-79606
 Validate extension JSON: Error: Field 'classes/RenderingDevice/methods/shader_create_from_bytecode/arguments': size changed value in new API, from 1 to 2.
 
 Added optional argument. Compatibility method registered.
+
+
+GH-78266
+--------
+Validate extension JSON: API was removed: classes/FontFile/properties/fallbacks
+Validate extension JSON: API was removed: classes/FontVariation/properties/fallbacks
+Validate extension JSON: API was removed: classes/SystemFont/properties/fallbacks
+
+The property was moved to their common base class Font.
+The setters and getters were already in Font, so this shouldn't affect compatibility.