This is something that can probably be implemented using the tools already built in, but it would be way more convenient to have it done automagically. In order to reduce clutter in the boards, create a duplicate of each forum that's hidden, and has posting disabled and editing disabled, but other than that has all the same permissions of its corresponding forum. All of the duplicates are hidden on the main index, but can be linked to in the footer. As threads are closed, grow stale, hit their length limit, etc. moderators can shuffle them off to the archive, where they can be viewed for reference but don't make clutter on the main page.