Actually, what a lot of the older stuff do is crop out the top and bottom. It's not real 21:9 support. You''re actually getting less of the intended image on screen.
Easily done but putting something like the corner of an object in the top left corner of a 16:9 image, thhen change to 21:9 and check if the view has expanded beyond the corner of the object in either height or width. You should take a picture, and possibly restart after changing the res, as some games need a reboot to properly engage in 16:9 to 21:9 if supported.
You don't need a 21:9 screen to test 21:9 resolutions. Just set custom resolutions if your displays support arbritrary resolutions.
Like on a 1080p screen (1920x1080), you can change to 1920x810 or even 800 or so. For a 2560x1440p screen (1440p), you can set to the ultrawide res of 2560x1080 (which many ultrawide monitors are sold as today). For 4k/uhd (3840x2160 screens), you can set to 3840x1600 which is the newest ultrawides sold today).
In third person games, you can easily tell it's fake ultriwide because your character's body gets more cut off instead of actually expanding your extreme left/right fov.
So yes, make sure it's not just cropping the image from what you can see in 16:9 mode. One example of fake Ultrawide is Castlevania Lord Of Shadows. I get less of an image. Now if I play Tomb Raider, I get the same image PLUS more on the left and right.
In fact, I'm positive you're cropping out the vast majority of those old games because they were made back when 4:3 was the main aspect ratio of video games. As monitors became more and more wide, more games started supporting actual wider fovs instead of cropping out to fit the majority of the image.
Let me give you some examples:
Here's DMC. It supports native 21:9. Disregard the black bars (as obviously I have a 16:9 4k display)
You see on the 1st image that is 21:9 how there's more to see on the left and right? More of the chandelier, more of the bottom of the stairs, while the top and bottom parts of the screen stop in the same places.
Now here's Remember Me, which does NOT have proper 21:9 support. This is how it would look like on your screen (minus the black bars, obviously)
Notice how her feet are cropped out, the top of the screen stops just past the neon sign in 21:9? And how the lest and right sides stop in the same parts as the 16:9? You're getting LESS of an image = not real 21:9 ultrawide support. Just cropping trickery. Real 21:9 should expand on what you see vs 16:9, not the other way around.