I was wrong about not being able to link to individual posts (see MaryFromMichigan's post). I've never tried to link to very old threads, but I'd expect the thread and post addresses to stay the same (or if there's a change, to have a simple system of updating the addresses).
That’s what I thought, that the links don’t change, even after many years.
If you save the link to a post, it will keep working.