So HDMI 2.1 had been blocked by the HDMI forum, despite AMD having done a lot of work to try to persuade them that it should be allowed in the open source drivers. It's not clear to me why the forum was blocking it, although a comment on Phoronix says, "Which companies are shareholders in this organization? Ah...MS. Apple, Nvidia, Broadcom etc... Hummmm !!!"

Valve is rumoured to be behind some of the recent work on AMDGPU, motivated by the Steam Machine - a different comment on Phoronix says, "When the Steam Machine benchmarks came out, showing performance inferior to the PS5, I got the impression it was limited by software rather than hardware", which sounds plausible. Getting HDMI 2.1 working on the Steam boxes is likewise probably a priority for them, as some gamers won't switch away from Windows until it's there.

It's not mentioned in the Phoronix write-up, but I presume the implication is the HDMI forum has relented and finally given their blessing to this work, rather than Valve is just YOLOing it.