I would imagine they are archived. Why?
Just wondered as I was surprised when i discovered that I had made 0 posts since my account was created in 2006....I remember taking an active role in the Office2000 problems of the 06-07 release and wondered what had happened to all those threads.
There was an issue when the board migrated to this new host (Jive) from vBulletin in that most peoples' post counts dropped to 0, mine included (with my post count I was particularly miffed...LOL). They had to be manually padded, which can be done with yours if you wish. You'd have to contact the forum administrator regarding that by posting a new discussion in Community Feedback.
As far as old posts disappearing that happens when the forums for products that are more than 2 years old are archived, but the post count should, theoretically, remain the same.