Sky Force Anniversary

Run the Game, select resolution 2560 x 1440, press ALT + TAB to get it windowed, exit the Game.
Open registry --> [HKEY_USERS\S-1-5-21-2155245860-1154593641-1153711486-1001\SOFTWARE\Infinite Dreams\Sky Force Anniversary]
put under "Default Screen Width_h1445351396"=dword:00000d70 for 3440
Run the Game and ALT + TAB again.

or hex edit:

Hex Edit the Sky Force Anniversary.exe -> look for 39 8E E3 3F and change the to CD 90 18 40
After that the 21:9 resolutions will be available in the Game Options.