Seems the hope for Easy Anti-Cheat being fully hooked up for Halo: The Master Chief Collection on Steam Deck and Linux desktop with Proton is not lost.
343 are actually working on it, but it seems like certain other games it has not been a simple case of just flicking a button and putting the files in the right places. On Reddit in reply to someone saying it's just ticking a box a 343 employee replied:
It's a bit more involved than that, but let me check with a few folks and try to get back to you with a status update.
Then in a follow-up post they said this:
Here's something more official from the developer on my team working on it.
We continue to work with Valve and EAC to enable EAC support on Steam Deck for Master Chief Collection. The addition of the easyanticheat_x64.so file is one of the prerequisites to getting this working, but there are ongoing compatibility issues that our three teams are collectively working to resolve.
I would love to provide more specifics, but the ongoing compatibility issues mean we only have a few pieces to the overall puzzle as everyone's working through their side.
Can confirm we ticked the box.
So there you have it. While it doesn't work right now, they are actually working on it. Once I hear any more on it, I'll be sure to let you know. You can play single-player offline though in Halo MCC, although you probably still need to do my fix.
At least Halo Infinite works though right?