Excellent idea! For a practical approach not bringing much overhead to anybody, what about:

0. If you are not a moderator, post the URL of a deprecated thread in a specific... thread to be created in the maemo.org forum.
1. Close the thread.
2. Add [DEPRECATED] in the subject. (only moderators)
3. Ideally: provide a notice and a recommended link in red font at the top of the first post: "THIS THREAD IS DEPRECATED. PLEASE VISIT <whatever>". Not always easy to do, but a deprecation implies that there is something valid and more recent.
4. Move the thread to the closest option in Old (Troubleshooting, Announcements, Newbie are good candidates to take most deprecated threads).

Next.

fragos, as I see it a thread is not deprecated because of belonging to a specific release, but because the thread itself has a more recent version somewhere else or goes around an issue that is not an issue anymore.

For instance, the OS2006 threads are not deprecated by OS2008 threads, and these are not deprecated by Maemo 5 threads.