Nothing has been deleted. You can see from the forum homepage that all 150,000 threads and well over 3 million individual posts still exist (and you can access any of these using the search feature).
As above, in the lesser used forums, you need to change the "Display Options" at the bottom - I think by default only the last 75 days worth of threads are displayed. You can open this up to show everything though. I'll see if the default can be changed in the admin console as I agree, it does make the less popular sub-forums look a bit unloved!