I'm not certain because I don't use Focus Mode when I play, but I think I heard that chess.com has had a recent bug where Focus Mode wasn't working properly, but chess.com is already aware of it and trying to fix it.
If you ever want to report potential bugs though, you can report it by the sidebar-Help-Report A Bug.
https://www.chess.com/forum/view/general/how-to-contact-support
I have my settings on to always use Focus Mode. The last few times I've played however, it wasn't on Focus Mode. I tried resetting it between games and during games, and nothing changed. (To wit, I ostensibly changed it to 'Focus Mode' in the middle of a game, and nothing changed.)
Am I missing something? The Focus Mode button is still there, so it doesn't look like they 'got rid' of it. Is it just not working, and if so, is there some way to bring it to their attention?