HTTP 451

[8] Some sites have previously returned HTTP 404 (missing) or similar if they are not legally permitted to disclose that the resource has been removed.

HTTP 451 was mentioned by the BBC's From Our Own Correspondent programme, as an indication of the effects of sanctions on Sudan and the inability to access Airbnb, the App Store, or other Western web services.

[14] When an entity intercepts the request and returns status 451, it should include a "Link" HTTP header field whose value is a URI reference identifying itself.

[15] At an IETF hackathon, participants used a web crawler to discover that several implementations misunderstood this header and gave the legal authority instead.

[16] The meaning of "a resource which cannot be served for legal reasons" has been interpreted to extend beyond government censorship: Media related to HTTP 451 at Wikimedia Commons

A 451 status code returned by the Defense Distributed website to a client in Pennsylvania, 30 July 2018. [ 1 ]
A 451 error message being displayed when visiting a website in mainland China
An American website not served to European users to avoid compliance with the General Data Protection Regulation .