IESG (Internet Engineering Steering Group), the group behind internet standards, have come up with a new error code to distinguish content blocked due to government censorship.
Historically, the status codes from 100 to 500 indicated technical errors. Most of us are familiar with 404, for example. However, there was no way to specify a non-technical reason like legal issues. Hence 451.
Govt. censorship of questionable material on the web is common nowadays with e-social giants like Facebook and Twitter receiving such requests everyday. Hence, the new error code makes sense.