It wouldn’t be HDMI 2.1 because it couldn’t be certified. And if you claimed it was 2.1 I imagine they would sue you.
Could it actually be made? I kind of wonder that. Like if one of the things you have to do is claim to the other device that you’re 2.1 would that get you in trouble? Or if you just advertise all the features and they each work is that good enough?
They could just say "we believe we're compliant with HDMI 2.1 but are not officially certified". No lies, no claims they can't make, and nothing I can see that would introduce legal risk to folks unless there's some patent encumbered garbage in the spec.
generally if something is needed for interoperability the courts only accept patents as a way to protected it (patents have a limited lifespan). However the law gets really complex and you need a lawyer for legal advice.
I think in this case you still couldn't claim it was certified. It would be on users to discover that if they plug an HDMI capable screen into that HDMI shaped port on your widget device, things just work and video shows up as expected
Note that if the protocol itself only works if the device claims certification you may be able to claim certification in the protocol. However you couldn't claim certification in marketing or any other context except where things wouldn't work if you were not certified.
Could it actually be made? I kind of wonder that. Like if one of the things you have to do is claim to the other device that you’re 2.1 would that get you in trouble? Or if you just advertise all the features and they each work is that good enough?