Undeclared dependencies to mw.util, mw.notify etc need to be explicitly loaded after the deprecation of wikibits (T122755).
Even if it is going to be temporarily reverted, I'd expect that we will have to eventually make these undeclared dependencies explicit.
Suggested fixes
See mw:Help:Locating_broken_scripts, mw:ResourceLoader/Legacy_JavaScript and mw:ResourceLoader/Migration_guide_(users) for more.
For Common.js, a typical fix with mw.loader.using will look like this:
(function() { ... x = mw.util.getParamValue( 'withJS' ); /* util is used but not loaded */ ... }());
mw.loader.using(['mediawiki.util']).then(function() { ... x = mw.util.getParamValue( 'withJS' ); ... });
For gadgets, typically the right fix is to add necessary dependencies into the dependencies= option written in MediaWiki:Gadgets-definition. If you are not sure you can continue maintaining the script, you might want to replace it with a reference to the same (or similar), better-maintained script hosted elsewhere: see https://en.wikipedia.org/wiki/Wikipedia:User_scripts#Full_manual_instructions for how to do that.
