Oops... my bad. In my earlier comment I assumed that this would be a Fedora/Ublue based distro, rather than an Arch one. Arch doesn't have RPM ostree either (which makes me dislike it as a choice for an immutable distro).

But, it's highly likely that with the steam deck and other projects, there is already an ecosystem for immutable Arch, and a minimal base system to start is advantageous, as Possibly Linux said.