WordPress vs. Discourse is quite useless comparison because tech and idea are so different, but on WordPress it is quite trivial job to use a free plugin, that calls outbound links and if response is something else than 200 OK it cleans at least links given 4xx status, makes a list and informs admin via email.
Is that even possible on such apps as Discourse, I don’t know. Is such functionality needed… I don’t know, because nobody cares anymore and saying good links never die has changed to all links will turn to zombies. Wikipedia is full of dead links, and no one cares, not even Wikipedia nor editors.
We have such thing called as flags. We can outsource informing to users and teach them how to use flags informative way — but I bet it will be hard.
Let’s be honest. Quite few admin/moderator is willing start fixing links that is never used after publishing. So ”link tester” would be nice to have, but is it useful tool? My biggest WordPress site has tons of dead outbound links. Those should be fixed and I will if I need those, but otherwise… I just don’t have time (or will…).