This is a simple temporary workaround for people having trouble with looking up old forum posts and getting Page doesn’t exist error (mainly using search engines).
Open your web browser and create a new bookmark.
Right-click on the bookmark and Edit the URL/ Location with the following line. (Optional: Change Title / Name)
Now when you get the Page doesn’t exist error, just click on this bookmarklet.
PS: This is just a temporary fix till search engines index those pages.
Edit: As mentioned by @papajoke, regex breaks on some browsers. So the script has been modified accordingly.