This is entirely down to the HDMI Forum, who blocked AMD from supporting 2.1 on their Linux drivers. There's nothing Valve can do until HDMI changes that decision, please point your pitchforks at the people responsible
www.phoronix.com/news/HDMI-2....
phoronix.com
HDMI Forum Rejects Open-Source HDMI 2.1 Driver Support Sought By AMD
One of the limitations of AMD's open-source Linux graphics driver has been the inability to implement HDMI 2.1+ functionality on the basis of legal requirements by the HDMI Forum