But have been wondering why I haven't heard of any immutable distros from arch based distros yet
SteamOS running on Steamdeck is Arch and immutabl/atomic for anyone not familiar.
Another one is blendOS
blendOS keeps everything simple, delivering on application and game compatibility from various sources while offering a lightweight atomic & declarative Arch system.
From their FAQ wording I get the impression it is disabled by design.