Can't help you with that one but I can tell you how to keep from losing threads in the future.
When you find a thread you think you might want to re-visit some day (including the threads you start yourself) put it on your "Watch List". At the upper left corner of the thread under "Topic Options" is a selection to put it there. Under "My Stuff" is the place to recall it.
Sorry not much help, but if you ever do find it, that is the way to keep it.
If you happen to know the name of the member who posted it, if they did not do an excessive number of posts, you can look up that member and view their posts. If you can remember any key words a regular search engine search of only the forum sometimes works. I can never remember the syntax to search only a specific web site though.
Edit: some search syntaxhttps://duck.co/help/results/syntax
Example search syntax to search only the forum:
Pond house over water site:forums.pondboss.com
site:forums.pondboss.com being the syntax that forces the search to a specific web site. Aren't old posts under another url? From the old forum archives?