It took quite a while for the issues to be figured out but Halo: The Master Chief Collection (MCC) now has improved Steam Deck support. This should also mean it works better on desktop Linux too.
While the game did somewhat work for a while, you weren't able to play fully online with others due to problems with Easy Anti-Cheat. Whatever the issues were have been partially solved now so players on Steam Deck can jump into EAC enabled games in multiplayer matchmaking and the Custom Game Browser. In a support post on the official site, the developers said that future updates will "continue to improve Steam Deck compatibility as well".
Testing it myself and yup — playing online works just fine:
There's some known issues though, like the launch options being backwards, that you need to keep in mind for now until future updates:
- When launching Halo: The Master Chief Collection on Steam Deck, selecting one of the available Anti-Cheat launch options will result in the game launching with the opposite launch option. For example, selecting “Anti-Cheat Disabled” will result in MCC launching with Anti-Cheat enabled.
- Until this issue is resolved in a future update, players on Steam Deck must select the “Anti-Cheat Disabled” launch option to access multiplayer matchmaking and the Custom Game Browser.
- Shortly after being removed from or leaving a party with other players via the Roster menu, players on Steam Deck may experience Halo: The Master Chief Collection freezing or crashing.
- Players on Steam Deck are unable to play Campaign Co-op or Spartan Ops with players on PC or Xbox consoles. Attempting to do so may result in players accessing gameplay, however, all sessions will eventually experience a “Connection Interrupted” error or a similar disconnection.
- Campaign Co-op and Spartan Ops will function as expected if all players in the group are playing on Steam Deck.
By "PC", they of course mean specifically Windows in the above quoted list.
For people trying it on Steam Deck for the first time, just be aware it won't automatically bring up the on-screen keyboard to sign in. You need to press STEAM + X to have it appear, and it may take a few taps in the input box to have it actually get focus from the keyboard. Hopefully they work on that too.