Some are taken down by the webmaster; some are taken down by the OP.
I can remember a time when there was a thread on which people were posting on a weeked, and and almost the entire thread became very political. I suspected that come Monday morning it would be gone, taken down by the webmaster for violating the rules, and Monday it was gone.
On the other hand, I have seen threads that I believe were taken off by the OP, particularly a post about an itinerary that I think the poster put up to show how smart they were, as in how many famous places can you "pass through" (no time to really visit) in a short time. I think Frank II once refered to such an itinerary as "blink and run". Most of the responses told the OP that the intinerary was not realistic. After too much criticism, that thread disappeared. I think that poster took their own thread down because they didn't get the kudos they wanted.