According to a recent Xbox Game Studios post on Steam, the studio has listed all of the verified and playable titles on the Steam Deck. There are several titles that got verified and they are usually Triple A games.
There are also some playable titles, but have not been verified. At least they can be played on the PC handheld, but they should be verified at a later time.
Surprisingly though, there are four titles that are unsupported. These are the titles in the list:
Microsoft Flight Simulator X
It was surprising that Halo Infinite was included in the list since it is one of the big games of Microsoft and a triple A game as well. So why is it not supported? Xbox Game Studios simply said it is unsupported due to anti cheat. According to Digital Foundry’s Rich Leadbetter, this also covers the game’s campaign mode, which covers most of the whole game.