I have done some testing, and 4:4:4 at 60 Hz can apparently only be selected on the Apple TV when you turn off HDR.Does HFR from iPhone 12 still play in 444 on the new AppleTV?
I don't know if Airplay can stream without chroma subsampling. I doubt it though (remember that it sends compressed video, since the bandwidth over Wifi is much smaller than over HDMI).