it's not even aarch64. but what they're saying is they don't want to deal with the support nightmare of supporting anything but the unmodifiable SteamOS image.
I think their point is: don't complain to them if you have an issue, unless you can reproduce it on the Steam Deck.
They don't want to deal with esoteric Linux bugs.
Every distro is a bit different though. And there's kernel / libc versions and the whole gui server on top of that. Windows gives you a few configurations to check, Mac does as well. But Linux means hundreds of possible setups before you even get to hardware differences. They just don't want to deal with that.
Steam runs all games in a container called the Steam Linux Runtime, so the only difference is the kernel and host Mesa drivers.
Not to nitpick, there is a 'native' option. Atleast it has been available on Arch for many years now (when SteamOS was on Debian?). In most cases we just the symlink the newer versions of libs to the older versions and the games run fine / better.
huh? but Steam Deck is just normal Arch Linux with x86_64 ~~aarch64~~?...