Update: Posted on X, the Halo Support account mentioned they're looking into it - "The #HaloMCC team is investigating reports that players on Steam Deck are experiencing Easy Anti-Cheat error messages after Tuesday's update. Stay tuned for updates on this bug investigation."
Original article below:
After a recent update Halo: The Master Chief Collection seems to have broken online play support for Steam Deck due to Easy Anti-Cheat.
If you remember, back in April they did announce Steam Deck support for online play and put up a support article for it. This came with a few minor issues, like needing to pick the Steam launch option that actually says it's off (the options were backwards). Since the update on September 19th, it just doesn't work with Easy Anti-Cheat errors.
When you go to launch multiplayer from the main menu with the option that has anti-cheat turned on you'll see an error like this when you try to go into multiplayer:
And when picking the Steam launch option with anti-cheat off, which going by their support article is still the option to actually use to have it enabled as it's backwards, you're met with this:
Going by the error messages, it seems like they may have put the Steam options actually the right way around now but broke it in the process. I've tested this myself across both Proton 8 and Proton Experimental and neither works. Restarting it doesn't change anything, rebooting the Steam Deck doesn't help either.
Lots of people on Reddit have noticed it and report the same issue (but a few note oddly it still works for them?), so hopefully Microsoft / 343 will fix whatever they did soon. I've reached out to their teams to see what's going on - will update if I get any reply.