The HDMI Forum has rejected our proposal unfortunately. At this time an open source HDMI 2.1 implementation is not possible without running afoul of the HDMI Forum requirements.
That also includes money to upgrade, for example, display equipment in virtually every office conference room, classroom, home theater, etc. It took a long time to shake VGA in those settings and now that that's largely been dropped in favor of HDMI it'll be a tall order to chase after the next best thing with no benefit noticeable to 99.9% of people.
It's an older interface than DP and has "better" support for audio (I.e. all of those proprietary passthrough audio formats that home theater setups support) so it became dominant in TVs. Monitors are still DP first but likely have a HDMI port as well.
In my experience, its cause monitors are already over priced, and adding a display port to it seems to add at least another 100 on top of that.
Which is why I prefer HDMI. Less cable headache too, since I only have to keep one type of cable in stock and so i can easily switch for testing/diagnostics/layout change purposes.
They don't need to RE it; they have access to the full spec and everything for their Windows drivers anyways. They'd open themselves up for litigation if they implemented this behind the forum's back though and that's something AMD (understandably) simply won't do.
Hopefully AMD start doing what Intel does and including a DP -> HDMI 2.1 converter in the card itself. There are already third party adapters that work reasonably well with existing AMD GPUs, especially on Linux. If they had their own implementation they could iron out the quirks and driver issues and get something that should be equivalent to real HDMI 2.1.