That is only true for old games from 10+ years ago. Every single game I've played, even indie ones, made after ~2016 either fully supports this aspect ratio or automatically adds black bars at the sides if it only supports 16:9. I've never seen a game fully render on the entire screen while at the same time UI elements are rendered out of bounds like this, besides incorrectly modded Bethesda games.
Thats the thing, its doing that because of that its not using the 21:9 ratio but the 16:9, but it results in that its UI is cut off at the bottom or top because of how it scale with the ratio.
Ive played a game that had similar problem, and the only way it could be resolved was that they needed to know where it scales when it try use 21:9 resolution types in order for them to readjust the UI to fit those resolutions.
