mod_rewrite (and possibly other modules) can't send a 451 error unless it is supported by the server. This is a basically standard patch to bring the status codes up to http://www.iana.org/assignments/http-status-codes/http-status-codes.xml (similar to #53929) Patch attached. Only question is about error page text - the RFC says it SHOULD be detailed about why the content is blocked and we can't really do that so easily in the default message.
Created attachment 33538 [details] trunk patch
Created attachment 33539 [details] 2.4 patch
Added to trunk Thx