diff --git a/RELEASE-NOTES-1.26 b/RELEASE-NOTES-1.26
index 896d1d583ef..852023bfc84 100644
--- a/RELEASE-NOTES-1.26
+++ b/RELEASE-NOTES-1.26
@@ -96,6 +96,7 @@ changes to languages because of Bugzilla reports.
 * The Block class constructor now takes an associative array of parameters
   instead of many optional positional arguments. Calling the constructor the old
   way will issue a deprecation warning.
+* The jquery.mwExtension module was deprecated.
 
 
 == Compatibility ==
diff --git a/resources/src/jquery/jquery.mwExtension.js b/resources/src/jquery/jquery.mwExtension.js
index e6e33ade111..548421286dd 100644
--- a/resources/src/jquery/jquery.mwExtension.js
+++ b/resources/src/jquery/jquery.mwExtension.js
@@ -1,9 +1,11 @@
 /*
  * JavaScript backwards-compatibility alternatives and other convenience functions
+ *
+ * @deprecated since 1.26 Dated collection of miscellaneous utilities. Methods are
+ *  either trivially inline, obsolete, or have a better place elsewhere.
  */
-( function ( $ ) {
-
-	$.extend( {
+( function ( $, mw ) {
+	$.each( {
 		trimLeft: function ( str ) {
 			return str === null ? '' : str.toString().replace( /^\s+/, '' );
 		},
@@ -14,9 +16,6 @@
 		ucFirst: function ( str ) {
 			return str.charAt( 0 ).toUpperCase() + str.slice( 1 );
 		},
-		escapeRE: function ( str ) {
-			return str.replace( /([\\{}()|.?*+\-\^$\[\]])/g, '\\$1' );
-		},
 		isDomElement: function ( el ) {
 			return !!el && !!el.nodeType;
 		},
@@ -28,7 +27,7 @@
 				return true;
 			}
 			// the for-loop could potentially contain prototypes
-			// to avoid that we check it's length first
+			// to avoid that we check its length first
 			if ( v.length === 0 ) {
 				return true;
 			}
@@ -117,6 +116,12 @@
 			}
 			return true;
 		}
+	}, function ( key, value ) {
+		mw.log.deprecate( $, key, value );
 	} );
 
-}( jQuery ) );
+	mw.log.deprecate( $, 'escapeRE', function ( str ) {
+		return str.replace( /([\\{}()|.?*+\-\^$\[\]])/g, '\\$1' );
+	}, 'Use mediawiki.RegExp instead.' );
+
+} )( jQuery, mediaWiki );