I always assumed every topic was still available (except those purposely deleted by administrators). However, I just went to the oldest topic available and it is from June 2003. I know that just within the past year I read topics from 2001. So I assume there is a limit to the capacity on this site.
It would be very difficult to judge which topics to preserve, so I assume when extra capacity is required the oldest topics are dropped. I suppose the lesson is that if you see a superb topic and subsequent discussion, you may want to print a paper copy for you own files. I personally have printed two topics in the past just for easier personal access in the future. Now I realize that after a certain number of years that may be my only access to those discussions.