Some of the original discussions around the status code referred to proxies but adoption at the moment seems to be mostly but hosting platforms like github, wordpress.com [1] etc.
I think the original ideal scenario was that a 451 would be generated by the in-country blocks that get put in place by ISPs due to legal requests from organisations.
However, they aren't incentivised to do this as much as hosting platforms are and so I don't think we will see large adoption there.
Instead, hosting platforms are using the status code both for DMCAs and other legal requests where the content may only be blocked for certain countries as part of a pragmatic response that keeps the rest of the service up in those countries (In country blocks are usually overly heavy handed :)).
I think the original ideal scenario was that a 451 would be generated by the in-country blocks that get put in place by ISPs due to legal requests from organisations.
However, they aren't incentivised to do this as much as hosting platforms are and so I don't think we will see large adoption there.
Instead, hosting platforms are using the status code both for DMCAs and other legal requests where the content may only be blocked for certain countries as part of a pragmatic response that keeps the rest of the service up in those countries (In country blocks are usually overly heavy handed :)).
[1] https://transparency.automattic.com/2016/01/05/error-451-una...