You can easily fix this by Hex Editing the UnityPlayer.dll File and changing all 39 8E E3 3F Hex Values with...
00 00 A0 3F for 1280x1024;
26 B4 17 40 for 2560x1080;
39 8E 63 40 for 3840x1080;
9A 99 19 40 for 3840x1600;
AE 47 A1 40 for 5292x1050;
AB AA AA 40 for 7680x1440.

Now you can enjoy the game in Ultrawide (even the Main Menu works!).

Keep in mind that this Fix needs to be redone after every official Update (also keep in mind to read the short Hex Edit Tutorial down below if you don't know how to Hex Edit).
If the Fix strangely doesn't work try to set the inGame resolution to 16:9 (1920x1080) since the 39 8E E3 3F Hex Values are for 16:9.

If you don't know how to Hex Edit:
Just download the program ''HxD'' from their Official Website. [mh-nexus.de]
--> Open HxD
--> Press Control O
--> Choose the Unity DLL File (''UnityPlayer.dll'' in steamapps/common/Still There)
--> Press Control R
--> Go to Hex Values (on top)
--> Search for 39 8E E3 3F and change it with any resolution values mentioned above
--> Activate Search direction all
--> Press Change everything (bottom middle)
--> (should change two values) Now go to the top left and save it where the normal DLL File is, changing it with the old one. If you have problems, just repair the game via Steam.