Just disclaiming that I don’t think there’s a “best practice” per se, depending on the context, if your intention as moderator is to direct conversation into “Topic X” that has the merged posts, you may choose to close the old topics (disallowing new posts) and add a post that issues related to “X” should go into “Topic X”.
That said, there are many ways to approach this. Topic titles that are more concise can help as well.