HTTP status codes are a core part of helping your browser understand what to do with a page. You’re probably familiar with 404 — page not found — but there are a plethora of others, like 302, which help tell your browser a page has moved.
The Internet Engineering Task Force (IETF) has finally created a standard for when a page has been taken down due to legal reasons. The new status code, 451, indicates that a host has received a legal demand to deny access to a resource.
This status code was added to help add “transparent operation” to the internet, and responses using error 451 should also include a explanation in the body including details of the legal demand, who made it and what legislation it related to, for example:
HTTP/1.1 451 Unavailable For Legal Reasons
Link: ; rel=”blocked-by”
Content-Type: text/html
Unavailable For Legal Reasons
This request may not be serviced in the Roman Province
of Judea due to the Lex Julia Majestatis, which disallows
access to resources hosted on servers deemed to be
operated by the People’s Front of Judea.
The response code is also intended to be used in cases where a provider in the access chain is blocking access, such as an ISP or DNS provider, to identify which entity blocked the page.
IETF Chair Jari Arkko said during the vote that he was in “FULL support of this specification” and the ballot passed unanimously.
Error 451 is approved for publication, but needs work from the RFC Editor, but Mark Nottingham, IETF HTTP Working Group Chair, said that “effectively, you can start using it now.”
Source: http://thenextweb.com/dd/2015/12/21/theres-now-an-official-http-status-code-for-legal-takedowns-451/