An HTTP Status Code to Report Legal Obstacles

...

Introduction

This document specifies a Hypertext Transfer Protocol (HTTP) status
code for use when a server operator has received a legal demand to
deny access to a resource or to a set of resources that includes the
requested resource.

This status code can be used to provide transparency in circumstances
where issues of law or public policy affect server operations. This
transparency may be beneficial both to these operators and to end
users.

[RFC4924] discusses the forces working against transparent operation
of the Internet; these clearly include legal interventions to
restrict access to content. As that document notes, and as Section 4
of [RFC4084] states, such restrictions should be made explicit.

2. Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

3. 451 Unavailable For Legal Reasons

This status code indicates that the server is denying access to the
resource as a consequence of a legal demand.

The server in question might not be an origin server. This type of
legal demand typically most directly affects the operations of ISPs
and search engines.

Responses using this status code SHOULD include an explanation, in
the response body, of the details of the legal demand: the party
making it, the applicable legislation or regulation, and what classes
of person and resource it applies to.

...

http://tools.ietf.org/html/rfc7725