A big portion of the improper overloading of HTTP status codes comes from WebDAV status codes seeming appealing when in fact WebDAV is a very specific set of functionality that is not really analogous to the way most REST APIs work... notably WebDAV offers locking semantics.
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.
Here’s the standard for 422: https://tools.ietf.org/html/rfc4918#section-11.2. It’s not a WebDAV-specific error condition. People are right to see the similarity and reuse the status code. This is not “overloading”.
There will be cases where an HTTP status code matches perfectly with an application specific scenario, and in those cases using an HTTP status code is not confusing.
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.
> In most cases it is clearer to simply adopt a payload format that includes application specific error codes
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.
A big portion of the improper overloading of HTTP status codes comes from WebDAV status codes seeming appealing when in fact WebDAV is a very specific set of functionality that is not really analogous to the way most REST APIs work... notably WebDAV offers locking semantics.