451 Unavailable For Legal Reasons
Censorship with a receipt: blocked by legal demand, and the number honors Bradbury.
RFC 7725, in 2016, created the code for blocks imposed by law — and picked the number on purpose: 451 is the temperature at which paper burns, in the title of Ray Bradbury's Fahrenheit 451. The response may carry Link with rel="blocked-by" pointing at whoever demands the block.
It showed up in court-ordered site blocks and, in 2018, on the American newspapers that chose to wall off Europe rather than comply with the GDPR. Transparency is the point: unlike the generic 403, 451 says why — and leaves the censorship's receipt in the protocol.
HTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://autoridade.exemplo.org/ordem-42>; rel="blocked-by"
Content-Type: text/html