Related: This is a nice write-up of how to write reactive jQuery. It's presented as an alternative to jQuery spaghetti code, in the context of being in a legacy codebase where you might not have access to newer frameworks.
I used this approach before and it indeed works better than the 2010-style jQuery mess. A good fit for userscripts too, where the problem you attempt to solve is fairly limited and having dependencies, especially with a build steps, is a pain. Note that you don't need jQuery for this at all, unless you are somehow stuck with ancient browser support as a requirement - querySelector, addEventListener, innerHtml - the basic building blocks of the approach - have been available and stable for a long time.
Unfortunately, nowadays writing userscripts is much harder than it used to be. Most websites are using some sort of reactive FE framework so you need to make extensive use of mutationObservers (or whatever the equivalent is in jQuery I guess).
Backwards compatibility. Apparently there are still some people stuck on IE11. It's nice that jQuery still supports those users and the products that they are still running.
I think anything still using ActiveX like stuff or "native" things. Sure, it should all be dead and gone, but some might not be and there is no path forward with any of that AFAIK.
jQuery made a messy ecosystem slightly less fragmented. Combined with CKEditor it effectively tamed a lot of web-developer chaos until nodejs dropped. =3
jQuery is v4 now, but a lot of sites esp. wordpress still have 1.11 or 1.12 and only uses them to either doing modals(popover), show/hide(display), or ajax(fetch).
I was surprised that for most of my smaller use cases, Zepto.js was a drop-in replacement that worked well. I do need to try the jQuery slim builds, I've never explored that.
https://css-tricks.com/reactive-jquery-for-spaghetti-fied-le...
Live on jQuery! Go forth and multiply!
Incredible it's still being maintained.
[0]: https://bundlephobia.com/package/jquery@4.0.0
[1]: https://bundlephobia.com/package/preact@10.28.2