Which of course makes no sense. Why would any action they take have anything to do with a website that isn't hosting their files.

Well, if the JS is to connect to an end-point on their server (eg. Google Analytics) if they remove/deprecate the old API the JS file won't work anymore as it might not be compatible with the new API, so the JS file would have to be updated.

Oh sure! I guess we're talking about different things. In your example, Google is providing you a service, and they're serving the file.

I was talking about using 3rd party file hosting services like cdnjs.com and unpkg.com

It's gotten so bizarre that highly popular libs (even Facebook's React!!! )will recommend you hotlink these third party sites rather than give you a zip file to download and host yourself.

Oh yeah, that use case is weird, and as another comment mentioned it no longer brings the benefit of caching.