The Group for Internet Standards, The Internet Engineering Steering Group (IESG), Approved a New HTTP code differentiate Web Pages which cannot be shown for Technical Reasons and Others which are not available for non Technical Reasons like Government Censorship.

Just like the Error 404 which tells the Users browsing over a page letting the User aware of the Page not found, So Example of Error 451 which Include A Web Page Considered to be a Danger to National Security or Web Pages Consider to Violate Copyright, Privacy or Any Other Law.
Source:- http://www.techmedia.com.ng/error-451-the-new-http-for-legal-reasons/

Informative!