There is extensive discussion about adapters in the bug report thread, it seems that some adapters do actually support HDMI 2.1 features, but it depends on their chipset and firmware version. I got a CableMatters one that seems to work, to the best of my ability to test without my TV having a diagnostic panel to show info about the signal it’s receiving.
https://gitlab.freedesktop.org/drm/amd/-/issues/1417