Is it known yet if the 9070 (XT) support HDMI2.1 in Linux
I hope AMD fixed this on their side, maybe by implementing it in hardware. Does anyone have a source? I've only seen reports of HDMI 2.1b support not specifying "Windows-only". Fingers crossed.
It's not known either way yet, but unless they offloaded it to a separate firmware like Nvidia or Intel, it's not possible for them to do in Linux due to HDMI patents. The way the other vendors avoided this is to use a separate component that does everything internally, so none of the code is in the drivers. AMD doesn't do that historically, and thus can only support such features with closed source drivers/OS.
TV manufacturers are part of the cartel HDMI forum, which benefits from there being a big ecosystem of non-member devices that are forced to have HDMI ln them and pay licensing fees for it.