There's nothing that says you can't have system messages as a boilerplate part of response bodies, even if you don't populate it.

In fact, it's the recommended way to do things like tell a client things it might need to know. Example: "The game servers will be shutting down in 15 minutes for maintenance."

True, but that means it has to be an ad hoc solution for every API instead of a standard.

It's not even an RFC, it's just a pastebin post pretending to be one.

Heh, after I posted it, a former co-worker forgot that was just an April Fool's joke and then, at his new workplace, seriously suggested handling one case by "throwing a 397".