HTTP is not a closed protocol. There will never be a single document defining all HTTP methods, status codes and such. Nothing in the definition of status code 422 makes it inapplicable to non-WebDAV applications. It is as much standard HTTP as status code 400 is (both are “proposed standards” in IETF terms).
I'm not arguing it is, it's an extension that adds specific semantics to accomplish a specific purpose.
But some developers look at WebDAV and see a lot of similarity with some of the application specific error conditions they are working with and decide that overloading HTTP status codes is a good idea, when it rarely is.
The confusion comes when the API developer tries to shoehorn all application specific errors into pre-existing HTTP status codes.
In most cases it is clearer to simply adopt a payload format that includes application specific error codes, so that the clarity of your API does not depend on its incidental similarity to pre-existing HTTP codes.
422 is a class of errors, not a catch-all for all similar scenarios one might encounter in building an API.
Absolutely. This is a common practice and there’s even a proposed standard for it (RFC 7807). But such a payload need not be sent with 200 (OK). It can refine the status code instead of overriding it.
True, this is sometimes possible when (coincidentally) one of the HTTP specs or extensions defines one of the codes in a way that feels similar enough.
The mistake is to assume that there is already a code for everything and that one does not need to define application specific error codes, which is what many API developers do.