I think that’s the only way to do it automatically (setting it to 0 disables the deletion). Though you can manually delete it at any point, including changing the timer length. It can be useful to have a record of the merge though, as it can be disconcerting for a user to think they have a topic, but it’s actually now somewhere else. They should get a notification, but I suppose it would depend on how savvy your users are in general?