The topic URL remains the same. So a bookmark to a post will still take you to the post no matter how many time it has been moved.
That was a horrible feature that cluttered the forum and gave an unfair officially sanctioned cross-post to the people who didn't bother to pick an appropriate forum category, pushing the topics from the responsible people off the first pages much more quickly. One of my favorite things about the switch to Discourse is that it stopped that practice.