> While often you will get a 404, this is wrong: the http call is successful. The endpoint did not vanish

According to RFC 7231, status code 404 means that the specified resource wasn't found. Not that the endpoint wasn't fount.

"The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists. A 404 status code does not indicate whether this lack of representation is temporary or permanent; the 410 (Gone) status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition is likely to be permanent."

So replying 404 is the correct response.

https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.4

There's.... There is a whole.set of status codes for exactly those things!

@number6 is right. There are all kinds of problems when you use HTTP status codes to represent something that was correct for HTTP but failed business logic. You don't want a tonne of errors logged because someone has violated business logic but otherwise called the API correctly otherwise good luck finding actual errors where you screwed up by deleting an endpoint (actual 404) or where you changed the request model (400) etc.

I'm sure people might disagree with that approach but it is very common and very reasonable.