It is but it is not really workable to use pacman. Each steamos major update breaks all installed packages as it has an immutable root that overwrites anything installed by you.
Most people just use appimages and flatpaks as they are installed on home so they are not overwritten.
I am not complaining. But it ought to have an overlayfs to keep at least /etc/ plus an file that keeps track extra installed packages, to be reinstalled easily.
There’s definitely no reason to install it on anything other than a handheld or PC being used as a console, though. It’s not officially available anyway - only way to install the version found on the SD onto anything else right now is via Holo ISO.