Support for HDMI 2.1 was the only reason I thought Apple would bring back the port. Egg on my face now 
It looks like they sacrificed one physical TB port for dedicated SD Card and HDMI ports running over the TB bus, instead of putting true HDMI 2.1 support that would have required 48Gbs to support all the resolutions and frame rates without compression. Ironically, you may be able to support HDMI 2.1 using a TB to HDMI dongle and DSC (Display Stream Compression). Have not confirmed this is supported yet.
It looks like they sacrificed one physical TB port for dedicated SD Card and HDMI ports running over the TB bus, instead of putting true HDMI 2.1 support that would have required 48Gbs to support all the resolutions and frame rates without compression. Ironically, you may be able to support HDMI 2.1 using a TB to HDMI dongle and DSC (Display Stream Compression). Have not confirmed this is supported yet.