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Valve: HDMI Forum Continues to Block HDMI 2.1 for Linux
(www.heise.de)
This is a most excellent place for technology news and articles.
To my knowledge they've never officially said but you can be sure that it has to do with Content Protection and that means DRM. An Open Source HDMI 2.1+ driver would make pirating much simpler, probably trivial and they don't want that.
It's possible anyway of course but there are a couple of hardware hoops to jump through and that's enough to keep most people from doing it.