By default the game comes with black bars on the side, but it does support 21:9, even though it doesn’t let you chose it. This is how you can force it to run on UW resolution, in my case 3440×1440.
Guide to Force 21:9 for Ultrawide
- You need to go into Settings> Graphics>change to Window>select 3840×2160 (or highest resolution that it lets you).
- Click Update and then click Keep Settings.
- Then switch back to Fullscreen Window and click the Update button once again.
- Everything should then be set to the monitor’s default resolution with no stretching.
Unfortunately once you exit and re-enter game, it’s back and you need to do it again! I hope they will add proper support for 21:9.