The steam client is only officially supported on Ubuntu and only using KDE, Gnome or Unity.
No. They offer a .deb package but repackaging and redistribution is allowed.
Valve’s support team won’t check issues on any other Linux setup.
That's false. I guess you never actually reported a bug on their Github page. Here is a random bug report from Steam on Fedora installed from RPMFusion with a Valve developer asking for details instead of closing the issue for being unsupported: https://github.com/ValveSoftware/steam-for-linux/issues/12422