Comment by breve

Six years ago Apple TV added support for playing 4K YouTube video:

https://9to5mac.com/2020/06/22/tvos-14-brings-support-for-st...

YouTube's 4K videos are only available in VP9 and AV1.

So the YouTube app on tvOS has supported at least VP9 for six years and I wouldn't be surprised if it supports AV1 today.