That's an quirk of how people think about HTTP response codes, which are defined confusingly enough already. They started out as server response codes but once

That's an quirk of how people think about HTTP response codes, which are defined confusingly enough already. They started out as server response codes but once people started making web applications and APIs they started overloading the server response codes to also have meaning as application response codes, making things even more confusing. Not Implemented is supposed to mean that the server does not implement the request verb (PUT, PATCH, DELETE, POST, etc.), not really relevant to anything that might exist or be running on the server. Maybe there could've been a separate standard header for application status, but that might not be great either since everything would have to handle all combinations of server and app status.