Consumers with newly purchased iPhone XS and XS Max need to swipe ans make yourself over to the app store and update your YouTube app, there you'll find the app will now support both these devices. Surprisingly, HDR support was only limited to iPhones 8, 8-Plus and iPhone X. Consumers can take full advantage of high dynamic range content.
The option to switch to HDR is there for late gen iPhones users only to be limited to viewing content in 1080p resolution. Android users such as myself have always granted the ability to see content up to 1440p with no issue. Even with Safari, the default web browser in MacOS has yet adapted to Google's VP9 codec which is customized for video resolution higher than 1080p, all while using less bandwidth.
How soon Apple will adapt to VP9, that remains to be seen. But for now, you're officially ready to view HDR content on the iPhone XS and iPhone XS Max if you right now update the YouTube application.