You are probably correct in assuming it's a thread that no longer exists. If it can't find the specified thread, it'll back up to the first part of the URL it
can find.
For the most part, threads are not deleted, as evidenced by the necro threads so often revived, going back at least 10 years. As an experiment, I brought up
this thread from 2003. So my guess is the thread you're looking for is indeed gone. You might try searching by the thread title instead of the thread ID, if you remember what it was.