MN can be found on the Wayback Machine.
If you enter www.mumsnet.com into the search, you can see caches of MN dating back to 2000, including the talk boards. This means that although threads on "Chat" and "30 days only" disappear from MN after a while, they're still viewable on the Wayback Machine years later.
It is possible to exclude your own website from the Wayback Machine. Is this something MN has considered?