This is a decent video for making a custom resolution.
https://www.youtube.com/watch?v=9II4djWfKq4
To disable scaling, you'd need to go into your Nvidia control panel. Then click "adjust desktop size and position" on the left side. Then click the small circle thing by the "no scaling" option in the scaling tab. That'll just make it so you can have black bars without it stretching to fill your screen.
To test it before you get a new monitor, you could possibly make another 21:9 resolution like 1890x810 just to see if 21:9 will work with black bars on a monitor of a different resolution. (Just in games that actually support 21:9, of course)
If it works, the new resolution 1890x810 should show up in the list of available resolutions in a game's options menu.
But in actual use ofc, on a 2560x1440 monitor, the custom resolution would need to be 2560x1080.