I'd say that coverage is very, very substantial, but incomplete because some games use anti-cheat that is either extremely invasive and heavily relies on Windows internals, or is anti-cheat that the devs have configured to reject running in Proton.
> basically-every-current-multiplayer-shooter is a big missing category.
Weird. I've been playing many multiplayer shooters from Proton with my Windows-using friends. I suppose this is one of those "am I friends with people who pretty much only play CoD or Fortnite?" things.