Actually it works fine on Steam Deck. It uses VRR over DP to the dock, which then translates it to HDMI with VRR. The dock has proprietary firmware to do this.
Intel and Nvidia hardware with open source kernel drivers also do a similar trick where the HDMI part is in a firmware blob. Only AMD does not work with HDMI VRR.
Actually it works fine on Steam Deck. It uses VRR over DP to the dock, which then translates it to HDMI with VRR. The dock has proprietary firmware to do this.
Intel and Nvidia hardware with open source kernel drivers also do a similar trick where the HDMI part is in a firmware blob. Only AMD does not work with HDMI VRR.