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.