The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve's Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.
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.