I haven't gotten around to trying the stable release out, but there's one ProtonDB report - presumably for 6.0 given the post date - so far that says it works flawlessly.

I'm guessing the situation is the same as pre-6.0, though. I participated in some of the playtests, where I got the same performance issues that I got before with the game slowing to a crawl after some time. The user on ProtonDB also has pretty beefy specs, so I couldn't say if the performance issues were fixed, either. I'm not sure if you're asking if it's playable or has gotten better, so I will say that (at least for pre-6.0) it technically works regardless as long as your computer is beefy enough.

The Proton GitHub issue for Squad might be nice to bookmark to make or check up on every once in a while; usually any issues, fixes, and updates end up there.