Crash Log: steam-311210.log
Just like this post, (https://github.com/ValveSoftware/steam-for-linux/issues/9061) I should have mentioned it crashes the whole steam client as well.
Additionally I also did mention of this issue in the Proton report of the game - https://github.com/ValveSoftware/Proton/issues/864
Your system information
Cannot play Black Ops III while using the Steam Client Beta. Game crashes immediately after seeing the main menu. My friend confirmed this happens on Steam Deck as well. If you go back to Stable it will work just fine.