I don't know of any threads that have ever been deleted simply because the members have lost interest. With the exception of a very few that took a really poisonous turn, they've simply been archived in the message tree, but remain open. The fact that some newcomers to ET occasionally revive a thread that was forgotten about two or three years ago bears that out.
Heh. Crashed out on dial up and Michael beat me to it.
Tarn, the board has an extensive archive. If you look at the forum topic listings and scroll down, you'll see archive links. For example, the Book forum has an archive of 980 posts over 108 threads, going back to late 1999.
Threads in archive sections (rather than those that are merely towards the bottom of the forum listings) are usually closed. To use the Book forum as the example again, the closed threads haven't been active for three years or more. They're easily reopened if the need/interest arises.