The 21:9 aspect ratio , 16:9, and 4:3 are all related. See this article : https://en.wikipedia.org/wiki/21:9_aspect_ratio . 4/3 squared = 16/9. 4/3 cubed = 64/27 or close to 21:9.
For a uhd sensor there is 3840x2160p, anamorphic pixel aspect ratio of 4:3.
There aren’t any 21:9 camera sensors according to this 2016 article: http://www.redsharknews.com/technology/item/1370-8k-is-not-the-future-of-tv-we-think-21-9-ultra-widescreen-is-instead?page=2 . It looks like that an anamorphic adapter on a 16:9 camera is the way to go to create 21:9 videos that can be edited and produced with powerdirector using PepsiMan’s technique.