The 451 HTTP error code, first proposed in 2012 as a tribute to Ray Bradbury's classic novel is now an IETF standard and is the preferred error message for a server to send to a browser when content is blocked for legal reasons.
The proposal was approved by the IETF HTTP Working Group, after a long wrangle over both technical and philosophical reasons not to adopt it. But some servers implemented it anyway, and reported that it was a useful in practice.
"By its nature, you can't guarantee that all attempts to censor content will be conveniently labeled by the censor," Nottingham explains. "Although 451 can be used both by network-based intermediaries (e.g., in a firewall) as well as on the origin Web server, I suspect it's going to be used far more in the latter case, as Web sites like Github, Twitter, Facebook and Google are forced to censor content against their will in certain jurisdictions."
There's still nothing stopping a government from forbidding the code's usage, however, which is a serious but perhaps unavoidable limitation.
The HTTP 451 Error Code for Censorship Is Now an Internet Standard