More and more phones are being released with 18:9 screens (can we just call them 2:1?), and OEMs usually make small tweaks to Android to improve the experience. Google is no exception, as the company has made at least one change for the Pixel 2 XL - a new mode for YouTube videos.
On the Pixel 2 XL, you can now pinch and zoom to adjust the video fit. By default, the video fits inside the screen, but you can do a zoom gesture to make it fill the entire screen. If you want to go back to the original fit, simply zoom out. You can see it in action below:
Other OEMs (like Samsung) have implemented their own solutions for making YouTube videos fill wider phone screens, but now it's built into the app. Unfortunately, the feature is currently exclusive to the Pixel 2 XL, so other 18:9 phones will have to keep using hacky workarounds. If you have a Pixel 2 XL and this isn't working for you, make sure you're on YouTube v12.40 and try force-closing the app.