With 301 to the other one.
Could that be automatic, similar way it should (is it?) when merging or by choose by acting staff because by definition it is 404; that can and should be under debate.
I feel doing redirections too difficult on Discourse, but the issue is propably just me.
At WordPress I have several plugins that helps me with 404/410/301/302 question. But WordPress is a bit different world per design and amount of users, so maybe it is unfair to compare.