Issue #3410301 by catch, diqidoq, Wim Leers: Deprecate system.performance stale_file_threshold

core/modules/system/config/schema/system.schema.yml

@@ -180,6 +180,10 @@ system.performance:
         gzip:
           type: boolean
           label: 'Compress JavaScript files.'
+    stale_file_threshold:
+      type: integer
+      label: 'Stale file threshold'
+      deprecated: 'The system.performance.stale_file_threshold config key is deprecated in drupal:10.2.0 and is removed from drupal:11.0.0. See https://www.drupal.org/node/3301744'
 
 system.rss:
   type: config_object