Actually 418 might be appropriate as the service was clearly not built to solve the problem as it exists today.
"The HTTP 418 I'm a teapot client error response code indicates that the server refuses to brew coffee because it is, permanently, a teapot."
IMO, "410 Gone" is only appropriate for URLs that contain unique identifiers because those are guaranteed to never be recycled.
Maybe they come back!
I would suggest is a 417 Expectation failed. https://http.dog/417
From RFC 9110, HTTP Semantics:
The 417 (Expectation Failed) status code indicates that the expectation given in the request's Expect header field (Section 10.1.1) could not be met by at least one of the inbound servers.
— <https://www.rfc-editor.org/rfc/rfc9110#name-417-expectation-...>