Although it was supported on iPhone X, so far YouTube did not support HDR on iPhone XS or iPhone XS Max, even though both devices could display HDR colors.
As first noticed by AppleInsiderApparently YouTube HDR apparently did not support out of the box for the iPhone XS or iPhone XS Max, but required an app update to enable this feature.
Unfortunately, Apple devices still do not support VP9, so iPhone, iPad, Mac and Apple TV users are stuck with 1080p video, with no options for 1440p or 4K, despite having enough powerful hardware to do this. support.
The update brings YouTube to version 13.37 on iOS and is available as a free update for all users.