It’s almost everything. You can play most games on Linux. You can’t bolt-on the quality of life features that Valve has on Windows.
There’s a reason most Steam Deck users don’t install Windows on it, even though you can.
I can’t speak for Epic Launcher games (I know that Heroic Games Launcher exists but I’ve not personally tried it with Epic games) however Blizzard games absolutely can be played in SteamOS - you can utilize something like Bottles or Lutris to install the Blizzard launcher, and then download the games from it as normal and run them. It is how I originally played Diablo 4 on my Deck before I picked it up again on Steam. I swear I remember both Bottles and Lutris even having an “Add to Steam” option to integrate shortcuts directly into Steam (and thus, coming up in the Gaming Mode UI) but don’t quote me on that one.
Blizzard games are actually some of the earliest non-Linux-native games that I remember running very well back in the days where we just had Wine (before Proton, DXVK, etc) which is something that always impressed me.
This is just my experience, but I have had next to zero issues running games on the Deck that were related to the platform. Most problems I’ve encountered are along the lines of the game being KBM-centric and it being difficult to play with the controller inputs.
The only Linux-specific issues I can’t think of are related to trying to install or mod games outside of Steam (Skyrim in particular is far more difficult to mod on Linux than I expected).
Like, can you run gamepass games on there and Blizzard games right from steamOS? What about games from the Epic launcher?
This is what I’m referring to when is say it restricts games you can play. Steam games mostly run pretty well.
Edit: Blizzard games do run
