After 10 days of waiting, 343 Industries / Xbox Game Studios finally again fixed up the online multiplayer for Halo: The Master Chief Collection on Linux desktop and Steam Deck after breaking it.
Their developers hooked up support for it originally back in April, and then broke it with the September 19th update that I reported on previously. Their official support account on X has now confirmed as of September 29th that it is fixed again and it seems they also finally after ~6 months fixed the launch options being backwards.
Steam Deck screenshot taken today playing online:
And a screenshot on desktop Linux (Kubuntu 23.04):
Both were using Proton 8.0-3 so it's working fine now.