I've found that if you bookmark a thread, you can still browse it after it has dropped off the list. Presumbably, you could then bump it back onto the list by adding a reply. As for the thread numbers, yes, you can dredge up old topics by entering the right number at the end of the URL. For example, this topic's URL ends in" 75703". Change that to "69741" and you get a topic from last February, "DATY: How common is it?" (Yay!) It's a crapshoot, though, and a pretty tedious one at that, because only about 10% of numbers yield anything, and there's no telling what that might be, nor is there a way to search for old topics by content. That's enough detective work for me. I think I will email Founder for further tips.
I agree, CG, that it's just as well for threads to disappear. My interest isn't in reviving them, but in referring back to choice bits of wisdom, humor, whatever. I guess it's a pack rat thing.