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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.
DP++ HDMI support tells the GPU to output HDMI. If the GPU can't output HDMI 2.1 over an HDMI port, it can't output it over a DisplayPort (as a general rule; you could theoretically wire the DP for a higher standard, but why would you?)