Although we didn't really see it coming, taller displays are becoming more the norm when it comes to smartphone hardware. As a result, the 16:9 widescreen aspect ratio won't fit in an 18:9 (or in Samsung and Apple's case with the Note8 and iPhone X: 18.5:9) aspect ratio without black bars. Some manufacturers have included a feature built into the UI that allows you to change the aspect ratio of the app so you can watch video without black bars. For those manufacturers that don't have such a feature baked into the UI, YouTube has added support for crop-zooming YouTube...
Source: https://www.gsmarena.com/youtube_updated_with_pinchtozoom_for_screens_wider_than_169-news-28290.php
