Previously most of popular phones on market had displays with 16:9 aspect ratio, and in this aspect ratio we were able to watch videos on YouTube. Now, when tendency is changing and majority of devices has displays with 18:9 aspect ratio YouTube will update their system and whole page will support videos for this type of screens.
After the latest update to 12.44 version on YouTube will appear new function which will allow us to fill all usable space of screen. Previously some phones had this function pre-installed into the system, but now this option will be available for all users which will watch videos on YouTube.