AMD is adding HDMI 2.1 support for Linux. That's good news for the Steam Machine.
Summary
Ars Technica reports that AMD is adding HDMI 2.1 support to the Linux amdgpu driver, enabling FRL and higher bandwidth features, with DSC to come later. This could unlock HDMI 2.1 features for Steam Machine and other Linux gaming hardware, though full compliance is still in development and licensing questions linger.