Bertram - it all depends on the individual thread in question. Some threads are just closed. In other cases individual posts are deleted and it's left open. In other cases the entire thread will be removed. It really just depends on the content of the message itself. I don't know of any posts that just were "no longer on topic" being removed. Very few posts get removed from the board (at most a couple per week). If there is a particular thread that you are concerned about why it was removed you can send a private message to that moderator to ask why or you can private message me and I will look into it. Sometimes mistakes are made - we're all human.
-Steve St. Laurent
Lead Moderator