Hi,
(If you don't use jQuery in your code or gadgets, you can ignore this email)
in 2017 jQuery 3 got deployed to production and a backward compatibility layer was introduced to prevent old codes from breaking. Since then it has been emitting warnings in your console. All start with "JQMIGRATE". Some of these old jquery breaking changes have been deprecated in versions released in 2006 or 2007.
We are slowly pulling the plug and removing that compatibility layer (which would reduce the default payload and make Wikipedia faster for everyone). Yesterday, that layer was removed from mediawiki CI, tests in gated extensions have passed but there is a chance that your extension might be broken in CI now if it's not part of gated extensions. In that case, please fix the errors.
We will move forward to deploy this in Wikimedia wikis soon meaning really old gadgets will also break if they don't fix their code. Please check your console and if you see jquery migration deprecation warnings, attend to them because they will break soon. Firefox doesn't give trace to warnings anymore but in Chrome (and using ?debug=1) you can easily find the source of deprecation warnings.
We already fixed many usages in code and gadgets and we will make sure to fix heavily used gadgets and user scripts but we can't possibly fix everything and it's the maintainers' responsibility to fix deprecation warnings that is being emitted for years.
Another aspect is that if you use jquery.ui, this library has been deprecated as well and I recommend moving away from it but if you still use it and migration can be hard, we patched our jquery.ui fork to make sure it doesn't break. Still, if you find a part of jquery.ui that has not been fixed, let me know.
Thank you and sorry for the inconvenience.