Since I don’t have any deleted topics I can’t test this, but I suspect you can go to the Title of the deleted topic and change the category to, say, uncategorized or to a hidden topic (maybe the better way so others don’t see) until you’re ready to retrieve them later. If you use a hidden category or topic, I would think you should undelete them when you move them to topic hidden category or topic.
AFAIK, re-categorizing a topic will update all posts’ and deleted posts’ URLs with the new category.