Is it possible to change iPhone X, iPhone XS, XS Max, XR, to display the screen in 16:9 aspect ratio? I think most of the iPhone X, XS, XS Max, XR have the aspect ratio of roughly 19.5 : 9. Is it possible to change the phone to use 16 : 9 aspect ratio?

Essentially, I think this means chopping off the top and bottom region, even just for an hour or two, and make it behave just like an iPhone 8 or 7.

One reason that is needed is for some apps, such as Pokemon Go, which are not "tweaked" so perfectly for phones that have different aspect ratio other than 16:9.  So when you catch a Pokemon Gastly or Kyogre, they are really hard to catch on the iPhone X, while they are not as difficult to catch on the iPhone 8 or 7. The reason is that the 19.5:9 ratio make the screen taller, and the Pokeball becomes very hard to reach the Pokemon, so the Pokeball will land an "air ball" and hitting just air. The Pokemon Gastly usually is not so important, but if you watch your friends catching the legendary Pokemon Kyogre fairly easily, and you throw and throw and get air balls or cannot hit the ring to get a Great or Excellent throw, and therefore your catch rate might not be even 50% of your friends, just because you are using a more advanced phone, that is kind of bad.
No, it is not possible to generally change the aspect ratio of the screen for any third party app.

You'll have to contact the app developers and let them know that their app has difficulties working correctly on these phones.