Halo MCC is now fully functional on Steam Deck
Thanks to a patch from the Halo: The Masterchief Collection support team, which was tweeted out on April 5, 2023
Halo MCC is now officially playable on Steam Deck with Multiplayer support
There is still a bug, which is noted in the support article linked to the tweet, that currently the Startup Anti-Cheat options are swapped, meaning to play with the fix and AntiCheat enabled, you have to start the game with the option that disables anti-cheat
No doubt this will be fixed in up-coming patch, but if you receive the anti-cheat error below
Then just start the game with the option you didn't pick the first time.
Although marked as Playable, there is still an issue on startup occasionally that will cause the game to freeze or show a fatal error, if that happens, just restart the game and you should be good to play.
We have had the error less using Proton Experimental instead of the standard Proton, but the error still occurred 2/5 times, so be aware that you may need to restart a couple of times.