There are literally N64 games with 16:9 support (Banjo Tooie, DK64, Perfect Dark, GoldenEye) and they still output a 4:3 signal. You can do ultrawide support without actually outputting an ultrawide resolution. You just expand the horizontal FoV in a way that looks squished in 16:9, but correct in 21:9.
Not saying the PS5 won't be updated with ultrawide output, just that it isn't necessary.
Banjo Tooie in normal 4:3 mode:
If you enable 16:9 mode and don't set your TV (or in this case the emulator) to stretch the image to 16:9, it looks squished:
So you set your TV (in this case the emulator) to stretch the image to 16:9 and voilá, it looks correct:
A 21:9 signal is not required in order to have 21:9 support in a game.
I'm too lazy to take these screenshots on my real N64 with my capture card, but trust me it's the same concept. You just set the TV to stretch and the image looks correct.