The Galaxy S8 and Galaxy S8+ are receiving HDR support in YouTube, according to some users on Reddit who have noticed the option to play videos in HDR popping up on their phones. This of course will only show up for videos that were shot with HDR in mind, and it appears to be a part of the latest update to the YouTube application on Android. That said this might not be showing up for everyone who has a Galaxy S8 or Galaxy S8+, and if that happens to be the case then it may be that YouTube still needs to be on the latest version.
With HDR video support for these two devices on YouTube, more devices which support HDR playback are likely to start seeing the option as well. Devices like the LG V30, and a few of Sony's latest phones also support HDR. In fact, the Xperia XZ1 now supports HDR playback in Netflix, as does the Galaxy Note 8, and the LG V30, according to recent reports that Netflix extended the support to those three new devices in addition to the phones that were already supported.
The option looks to be showing up for various models of the Galaxy S8 and Galaxy S8+ from different carriers, with users in the Reddit thread mentioning in the comments that they have T-Mobile and Sprint versions of the phones. Since this is an update to the YouTube application that is enabling this new capability for video quality, the carrier model of the device won't matter. If you've never messed with the resolution settings for videos inside YouTube on Android before, they're pretty easy to find. Simply tap on the overflow menu button in the top right corner of the video and this will bring up a list of options at the bottom of the app UI. There, you'll see an option for video resolution, and what the current resolution is that the video is set to. Tapping on this will bring up the list of resolution options that your device supports as shown in the image below. If your device supports HDR in YouTube, you should see HDR next to the resolution options.