451 Unavailable For Legal Reasons

The resource cannot be served for legal reasons, such as a court order. The number is a nod to Fahrenheit 451.

Class
4xx Client Error. The request has a fault, or cannot be met as sent.
Registered
Yes, in the IANA HTTP status code registry.
Cacheable by default
No. A cache stores it only when Cache-Control or Expires says to.
References
RFC 7725 ยง3 · MDN

Example response

HTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://example.com/legal-notice>; rel="blocked-by"

Every 4xx code

All HTTP status codes, in one table