With the large number of users who use YouTube from their mobile phones and with the large number of vertical videos, YouTube decided to give up on these videos and now plays them in full screen without the need for black bars. In this way you can enjoy the video in the same way in which it was recorded, vertically.
This function for now only works on Android in version 10.28 of the YouTube application that will arrive on Google Play soon.
As much as Google blocks the ability to record vertical video in its camera app, dedicated music videos, and videos preventing how horrible vertical videos are, YouTube has to give in to this horrible trend, but now we can see more types of videos. full screen content on our cell phones.