Is it just because they are old? Is this done automatically?

Really, some of these are being deleted and they contain info that is usable. Keeping them up will keep some of the endless repeat questions from having to be endlessly answered.

Quit deleting threads because they have been dormant for a short time!!

Old threads are not being deleted automatically, sometimes its just hard to dig.. even using different key words. A post is deleted when its be threadjacked or is not going anywhere helpful in response to the original discussion. Redundant threads that has been constantly discussed are also deleted.

The new software doesn't keep the same number of threads up top. 3-5 pages at most. Unfortunately, it requires more searching.

Check the "Display Settings" at the bottom of the page. By default, it shows threads from the last month only. You can change it to show older threads in the dropdown box.