Quick reminder on the likely reason why Valve's New Steam Machine only supports #HDMI 2.0:
""At this time an open source HDMI 2.1 implementation is not possible without running afoul of the HDMI Forum requirements.""
This is what Alex Deucher, the maintainer of the amdgpu #kernel driver, said one and a half years ago here:
https://gitlab.freedesktop.org/drm/amd/-/issues/1417#note_2303163
And from the ticket it looks like the problem remains.
See also Alex's earlier comment from early 2021 in the ticket:
https://gitlab.freedesktop.org/drm/amd/-/issues/1417#note_830547
""HDMI 2.1 is not available on #Linux due to the HDMI Forum.""
