In brief: It appears anti-cheat is keeping some popular games from being playable on Valve's Steam Deck. First, it was Fortnite, and now Microsoft has confirmed some of its online games face the same obstacle, though it did note compatibility for many other games.
This week, Microsoft laid out a selection of a little over a dozen of its recent games it confirmed are at least playable on the Steam Deck. However, the company says Gears 5, Halo: Master Chief Collection, Halo Infinite, and Microsoft Flight Simulator X aren't compatible. It simply states "due to anti-cheat," with no other details.
However, last month Epic Games explained why anti-cheat kept the Steam Deck from supporting Fortnite. Epic boss Tim Sweeney expressed doubts about fighting cheating across the wide variety of Linux kernel configurations, particularly the one the Steam Deck uses. Sweeney said Epic is working on making Easy Anti-Cheat more compatible with Steam Deck. Some Microsoft games may be facing similar issues.
The games Microsoft confirmed are fully verified for the Steam Deck are Deathloop, Psychonauts 2, Hellblade: Senua's Sacrifice, The Evil Within, Fallout Shelter, Prey (the 2017 release), Battletoads, and Max: The Curse of Brotherhood. Titles it lists as "playable" are Sea of Thieves, Fallout 4, Forza Horizon 5, Forza Horizon 4, Quantum Break, and State of Decay: YOSE. The latter list of games may require users to tweak some settings to play.
Microsoft has many other games on Steam that it didn't mention this week. The Steam page for Ori and the Blind Forest says the game is verified, but its sequel is unsupported. Fallout 3 is unavailable for Steam Deck, but the first two Fallout games are playable. More games like the 2016 Doom and the newer Wolfenstein games remain untested.
The nuclear option for running all these games would be to install Windows on a Steam Deck, which Valve just made more manageable by releasing Windows drivers for the device.