Index: /trunk/data/validator/deprecated.mapcss
===================================================================
--- /trunk/data/validator/deprecated.mapcss	(revision 14916)
+++ /trunk/data/validator/deprecated.mapcss	(revision 14917)
@@ -1404,2 +1404,11 @@
   group: tr("deprecated tagging");
 }
+
+/* #17504 */
+*[is_in],
+node[/^is_in:.*$/],
+way[/^is_in:.*$/] {
+  throwWarning: tr("{0} is deprecated", "{0.key}");
+  group: tr("deprecated tagging");
+  fixRemove: "{0.key}";
+}