Valve: HDMI Forum Continues to Block HDMI 2.1 for Linux
www.heise.de
Technically, the Steam Machine supports HDMI 2.1. However, Valve and AMD are not allowed to offer an open-source driver for it.

I do not really have a body for this. I was not aware that this is a thing and still feel like this is bs, but maybe there is an actual explanation for HDMI Forum’s decision that I am missing.