A Redditor by the handle of mynameisjefffffs posted a screenshot to Reddit, allegedly taken from a Google Pixel 3 XL, showing HDR being available in the Netflix app, indicating that the feature has begun making its way out to users without much fanfare. Many other users in the same thread stated that they had not received the feature yet, which means that it's most likely on some sort of slow rollout. The thread took place in the r/Google subreddit, which means that most of the users in the thread probably had Pixel devices of some persuasion.
The feature presented itself on the poster's phone with no real warning of any sort, and was not preceded by any kind of update or extra announcement. In fact, one other person in the thread reported having the same version of the Netflix app loaded up, but they were not presented with the option to view HDR content. This means that the rollout is server-side, so there's not much that Pixel 3 XL users can do except sit back and wait for the update to hit their devices.
Once you do have the update in place, it looks like HDR will be enabled on a per-item basis. When you go to launch a piece of content, you'll be given a notice that the content is HDR-enabled via a small HDR icon right below the large play button that features a preview of the content. You'll know that you've gotten yourself into a piece of HDR-enabled content with the feature turned on if you can't turn your brightness up all the way. That's a feature that's unique to Netflix as far as HDR content, it would seem; some users were reporting that they were able to crank up the brightness for HDR content on YouTube.
Smartphones and other devices with HDR-ready screens are not an entirely new phenomenon, but they're still plenty new enough that there's not a ton of HDR-enabled content out there just yet. Netflix is one of the parties looking to change all of that, and has been certifying devices left and right to display HDR content from its service.
The proliferation of HDR content is one of the best ways that Netflix can work to push ahead of the competition right now, and that's especially true for smartphones. Many people who do watch films on Netflix via their smartphones don't do it in outdoor, high-light, or otherwise distracting settings, and that means that the closer proximity to the viewer's face when compared to TV makes a good color and light balance experience that much more important.
Going forward, expect to see Netflix roll out HDR to an ever-growing stable of devices of all sorts, from game consoles and TV sets to smartphones and tablets. Google's Pixel 3 XL is in the spotlight with this story, but there seem to be scattered reports of the smaller Pixel 3 starting to get the feature as well, so those who've invested in Google's hardware can rest assured that they'll be able to enjoy high-definition HDR content in bed, the bathtub, or on their lunch break very soon.