I can't tell if I'm missing the point or this spec is.

To properly handle errors, there needs to be an exhaustive enumeration of the types of errors. This is hard to generalize across all applications and this RFC just sidesteps it entirely.

We don't need yet another way to bundle together a bunch of strings nobody ever reads and that applications will have to parse because there's no error code beyond the woefully inadequate HTTP status. If the app dev has to roll their own enumeration scheme and put this application-level error code in an extension, that's no better than just responding with the JSON they already had.