AMD wanted to incorporate parts of the HDMI 2.1 spec into its open-source Linux drivers, but the HDMI Forum's legal beagles shutdown the whole idea
If you're a Linux gamer and use an AMD Radeon graphics card, you might be aware that there are all kinds of limitations and bugs when trying to run at high resolutions with high refresh rates, when using the HDMI port. AMD's software engineers had been working on a solution to it all, by implementing parts of the HDMI 2.1 specification into the code, but sadly had to shelve the whole project because the HDMI Forum rejected the project.