A Plague Tale: Innocence is an amazing game which sadly doesn't support Ultrawide (neither Gameplay nor Cutscenes).
You can easily fix this by Hex Editing the Game and changing all 39 8E E3 3F Hex Values with 8E E3 18 40 Hex Values (for 3440x1440; 26 B4 17 40 [instead of 8E E3 18 40] for 2560x1080; 9A 99 19 40 [instead of 8E E3 18 40] for 3840x1600).
Now you can enjoy both Gameplay and Cutscenes (aswell as the HUD) in 21:9 (even the Main Menu works).
Sometimes the transition between Loading Screen and Cutscene glitches out with red bars on the side but it shouldn't be too worrying (since it lasts only for the duration of the Loading Screen).
Keep in mind that this Fix only works for Steam and needs to be redone after every Update (also keep in mind to read the short Hex Edit Info 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.
Main Menu:
Spoiler:
Cutscene:
Spoiler:
Gameplay:
Spoiler:
If you don't know how to Hex Edit:
Just download HxD --> Open HxD --> Press Control O --> Choose the Game's Exe (''APlagueTaleInnocence_x64'' in steamapps/common/APlagueTaleInnocence) --> Press Control R --> Go to Hex Values (on top) --> Search for 39 8E E3 3F and change it with 8E E3 18 40 --> 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 exe is, changing it with the old one. If you have problems just repair the Game via Steam.