The YouTube app for iOS has gained HDR support in the new update.
Version 13.37 of YouTube for iOS has added new quality options when viewing videos. Compatible clips will now show 'HDR' next to the resolution, showing the high dynamic range content. The Auto option also enables HDR.
HDR (high dynamic range) brings a wider range of colours in a video (like Apple TV 4K). The effect allows for a more vivid and brighter picture, too.
The OLED display on the iPhone XS and XS Max allow high contrast ratios, letting the user view HDR content on the screen.
The release notes of the update to the YouTube app do not state added HDR support, and YouTube has not announced it either. This is a welcome change for many iPhone X and XS users.
YouTube still haven't added support for resolutions over 1080p - there is no sign of 4K. The issue exists on the iOS app, but also YouTube for Apple TV 4K, and the YouTube website in Safari on macOS (it offers 4K if you use Chrome).
It's unknown if, or when, support for higher video quality will come to Apple devices.