It was blocked from inclusion in the AMD GPU drivers, it's nothing specific to Steam or the Steam Machine.
The HDMI Forum apparently forbids any open source implementation of HDMI 2.1. Although I don't know if they ever offered an official justification, for a group that exists to promote HDMI adoption, they're clearly morons.
This was previously blocked from inclusion in SteamOS by the HDMI forum. It would help the Steam Machine to each 4K120Hz on HDMI.
It was blocked from inclusion in the AMD GPU drivers, it's nothing specific to Steam or the Steam Machine.
The HDMI Forum apparently forbids any open source implementation of HDMI 2.1. Although I don't know if they ever offered an official justification, for a group that exists to promote HDMI adoption, they're clearly morons.