I have a similar question so I want to piggyback on your post. It seems to me that the camera FOV is pretty jacked up in 21:9. I forgot to take a screenshot before I left but the trapezoid on the min-map is much wider at the top and narrower on the bottom than the screenshots I'm seeing of 16:9. I poked at the cfg files I could find but didn't see anything obviously related to the FOV.
The game is playable in 21:9 and the UI is totally fine. It's just the camera angle that is bad, everything is tilted off into the background far too dramatically, I want to have more of a top-down view than I'm allowed. Hopefully it's a simple fix or something they address in a patch. Or maybe I'm just missing something.