What do you mean "resized to 21:9"?
I’m gaming on an LG C9 - which is a 4K 16:9 display, obviously. If you want to game in 21:9 then in the Nvidia Control Panel you can re-size your display to 21:9 by lowering the Height all the way down.
Essentially, you’re re-sizing your display to 3840x1620, which is 21:9.
How is this different than a custom resolution? Because doing this, for some unknown reason, doesn’t disable HDR. A custom 21:9 resolution will disable HDR.