Popularity of videos with unusual aspect ratio started when smartphones became everyday devices for most of society. Most of videos recorded with using of smartphones were vertical, so after uploading it to the Internet film was scaled to 16:9 proportions by filling the gaps with a black background. YouTube published information that firm dealt with this problem and videos with black background are a thing of the past.
Video player after new update of YouTube will automatically adapt to size of video - regardless of whether it is a vertical film or a standard one with an 16:9 aspect ratio. You can check this change by entering on YouTube webpage on computer, but in the nearest future update with dynamic size of video player will be released also for app dedicated for Android and iOS systems.