I actually use mlocate a bit, one benefit is that it's a simple system, easy to understand. And it's seen some use, so it tries to keep permissions consistent between the index/search and the filesystem.

There's also been a few attempts at integrating search with the various desktop projects, generally backed by xapian or some other full-term search library. I'm not sure what are currently the best/best maintained options.

I seem to recall gnome "tracker" had the most traction last I checked, not sure if eg the kde project has something similar. Looks like canonical booted tracker from the default install in 18.04 lts:

https://community.ubuntu.com/t/install-tracker-by-default-in...

Microsoft had its aborted attempt at a new fs built on top of sql server, for "proper" search at the fs level. I'm not aware of any real file systems that do full search out of the box. And I guess it's not clear that it'd be any better than initial indexing+index refresh on change/on a schedule.

KDE has Baloo [0]

[0] https://community.kde.org/Baloo