As far as I can recall: some internal implementation documentation leaked which made the legal position tougher for them somehow to protect it as proprietary (it made sense when I heard it explained, but now I feel like I'm missing an important detail to that), and then Valve had some talks with the HDMI Forum to convince them to let it go, presumably so they could have such support integrated into Linux for their hardware (Deck, Machine, Frame, etc).
So now the HDMI Forum just doesn't really care anymore I guess.
IIRC, a non-AMD contributor was working on adding HDMI 2.1 support to Linux's AMD driver, and then AMD managed to get the HDMI Forum to see that their position was moot and was making things very awkward for AMD.
Oh this makes sense! Obviously, it can't be illegal for a non-forum-member to add HDMI support; it's just an HDMI forum policy after all, not law. That's certainly the link I was missing for this to make sense.
presumably someone at Valve said "either you do this or we will pay an open source contributor 100k a year to reimplement every spec you make for the rest of time"
I actually like that reality better
It's not like the policy ever made much sense to begin with. Hard to try to keep track of what a rationale might be enough to convince people who are already being unreasonable!