You can kinda make it bend a bit to your whim. While it is technically immutable if you don’t mess with it, it does have everything necessary for using pacman. It just all reverts next time steamos updates. Anything you install directly through the discover portal is permanent, but it does technically have access to anything in the pacman repos as well.
I unlocked mine long enough to download neofetch and take the screenshot for this. It’ll revert back soon, but I only needed it temporarily for imaginary internet points. :)