I used Mylyn's focus feature quite a lot back in the day. And I think Mylyn was revived, I just tried and it still works. However, it only hides projects/files/folders in views, I don't think it was ever hiding code within a file (or I remember incorrectly). It is pretty cool until it is not. And if it is not helping, you are constantly manually resurfacing hidden parts, which is annoying. This made me turn off the part that hides items not used recently and only turn it on when really working on some isolated parts in a huge set of projects. The other cool thing about it is that it is coupled with issue management (which you could sync with your issue tracker) and you could store the "focus" (which items are currently visible) in the context of an issue. So if you switched working on issues, it would only show the items relevant to the issue again.

BTW: In Eclipse JDT you can configure what kind of Java elements should be folded by default. The default config is that leading comments and imports are folded. Would be nice to be able to configure something like that via LSP.