In particular, just as the body of a 404 or 403 response can provide information to the client about how to make the request successfully (e.g. "did you mean one of these?" or "you need to log in", respectively), the body of a 451 response could suggest what the client could do to make the request successfully (e.g. "please try again from a less broken jurisdiction or over a more secure path").

Yes, they even talk about recommending to the user to use Onion instead, so that's definitely a case they are considering.

Agreed. I think this would often be the case for returning a 451 as well.