There is a reason why Halo: The Master Chief Collection is marked as unsupported on Steam Deck by Valve. Namely, the anti-cheat.
First of all, you need to disable it in order to be able to launch the game. Secondly, it means you can only play campaigns, and only with achievements disabled.
And last, but definitely not least, the performance is not ideal. There are some FPS drops, and the game can occasionally hang. There are some FPS drops, and the game can occasionally hang. Using CryoUtilities (https://github.com/CryoByte33/steam-d...) can help a bit with the hanging problem, but it still doesn't play very comfortably even on the original graphics quality.
Image by Andrea Wierer from Pixabay: https://pixabay.com/users/andreaswier...