The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve’s Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.
They charge a fee for access to the spec and maintain who can claim their products are HDMI compliant and require compliance testing on those products.
An open source implementation would make that spec public and strip a lot of control they hold.