So far, nothing -- most are still here except in forums that are date-sensitive, like Last Minute Rentals and Sightings, in which out-of-date threads are removed. We did have an as-yet-unexplained incident in which all existing messages in TUG Lounge were lost about a month ago.
It will not be long, however, before we start running out of database space and will have to start pruning old messages. We've alerted Bill Rogers to our need for increased database space, but no good news on this front so far.
Since this question deals with BBS issues, I'm moving it from TUG General to TUG BBS forum.