January 2021 and the situation with the YouTube app on Apple TV 4K is this...
- If a YouTube video is 4K/30fps then it will play in 4K on the Apple TV.
- If a YouTube video is 4K/60fps then it will play at 1440p/60fps on the Apple TV.
- The YouTube app does not support HDR on Apple TV.
I personally believe this is all due to a restriction of the A10X Fusion in the Apple TV 4K. The iPad Pro 2017 10.5” also uses the same A10X Fusion. YouTube app is restricted to 1080p, no HDR on that iPad.