when we click on a old forum’s link in Google, we get a 404 page:
Oops! That page doesn’t exist or is private.
Perhaps, you could integrate the warning with something like:
If you have reached this page by clicking on a link related to the old forum, try the following link:
And then replicate the page address (copy it from the browser address bar) and postponing the word ‘archived’ to the address.
The user will click on the new link that will take him to the corresponding page of the old forum.
What do you think about it?