Every so often I prune the forums to remove clutter. Threads die and serve no purpose (ie perhaps it was created to fix a tech issue and the issue is now fixed), or it's imply outdated ("Happy Easter", "Merry Xmas", etc). So I simply throw them in the Vault sub-forum.
If you can't find a thread for some reason, it's probably there, although I doubt anyone would be looking for those threads, which is why they're vaulted.

Also, the development team has its own private vault for development threads that are outdated. It's called the Undermountain Vault subforum. Same idea there.
If anyone has any problems finding something, LMK.