I understand that some HTTP status codes are cacheable by default. From RFC 7231:
Responses with status codes that are defined as cacheable by default (e.g., 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501 in this specification) can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls [RFC7234]; all other status codes are not cacheable by default.
Why is HTTP status code 451 cacheable by default? From RFC 7725:
3. 451 Unavailable For Legal Reasons
... A 451 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicit cache controls...
451 is a new HTTP status code used for signifying a resource that is unavailable for legal reasons e.g. such as for copyright issues. GitHub uses it for DMCA notice'd repositories.
A resource that was legally unavailable yesterday might be made available today. I understand obviously that caches are short-term, so 404 being cacheable makes sense, but I would have thought that when it comes to matters of legal censorship, currentness would be of vital importance. Why then should this status code be cacheable by default? A response could specify when it wants the status code to be cached, when the responder believes that the resource will remain blocked for a while.