In Poohbah's case, he did. He notified us on multiple threads several days in advance that he was leaving and why. Many, including me, tried to talk him out of it. The only warning I had about hchutch was a freepmail about 2 minutes in advance of his opus.
If I leave this forum I doubt I'd do an opus, at least not as a separate thread. I've already had one, LOL. Another Freeper wrote and posted my opus as a prank about 5 years ago. Pretty funny.