Steam is supported on Ubuntu and Fedora. That gives it .deb and rpm coverage, enough for the rest of the downstream distros to make it work.

Anecdotally I've had great success with it on Fedora and Linux Mint (Mint being a spin-off of Ubunutu).

> That gives it .deb and rpm coverage

Package formats are mostly irrelevant for Steam. The only thing that is packaged is a tiny bootstrap script with the proper dependencies to make sure you have stuff like 32-bit system libraries installed. You don't even need to use that.