>>I'm sure this is covered somewhere on Acaeum, but what was the last product produced for First Edition AD&D?
Trick question: The answer is L3, 1999.
The first module I recall specifically stating that it was compatible with 2nd Edition was WG8 Fate of Istus, which I believe shipped in March of 1989. Notably, this was actually a 1st Edition module, so I believe this was the last one. See
Page Not FoundThe Player's Handbook (as opposed to Players) shipped in April of 1989, and the DMG in June, according to the "Coming Soon" bits in Dragon magazine and catalogs; but I don't recall seeing either until August (admittedly I was not in a big city at the time). So I suppose anything marketed for March 1989 release could also be a contender. (FR7 Hall of Heroes, 1st printing, perhaps? I'd have to look again. But it's a sourcebook, not a module.)