Perhaps the post was rehomed in a more suitable section and the shadow link in the original location has expired?
Some of the classifieds, Ebay and the like have a time limit on, so they disappear automatically after a certain period (30 days?).
One way of trying to track down an old thread that you posted on is to go into your profile and search through your posts. Even if a thread has been moved, you can find your own posts and locate the thread that way.
I'm not aware of any major clean-up taking place but it's possible
I have just tried the View Your Posts method and have noticed that they only go back to 09 May and my joining date was 01 Mar so there must be some gone.
I was also trying to view the posts by using the link in a PM that appears when someone PM's you from a thread and get the message something like "The topic you requested does not exist"
This is not an urgent matter and was only to find an old post but doesn't matter if its gone.