So after 5 years there's still no 21x9 support. I tried to configure it myself but failed.

Things i tried:

-That mod - Complete Widescreen Fix for Vanilla and SkyUI 2.2 by Daisuke

- I got a game running but when attempting to access inventory - it crashed

-That fix - Ultrawide Interface by InAComaDial999

-Manually changing Skyrimprefs.ini file

- game started 16x9 only