In a recent discussion following the question about edition transitions in the Forgotten Realms, the exact point when Karsus's Fall became part of the canon of FR lore was debated. When the Arcane Age was introduced in 1996, there was the boxed set Netheril: Empire of Magic, which detailed Karsus's Fall and how that caused Mystryl to reincarnate herself as Mystra, and there was also an adventure named How the Mighty Are Fallen, in which the player characters could participate in the final days of Netheril. However the adventure as it was written did not really give the opportunity for the characters to stop Karsus.
On the other hand, it might still be possible to argue that the whole Arcane Age scenario was an optional component, and the DM could entertain the possibilty of having her players stop Karsus, or even claim that Netheril was not even the way described in the boxed set. It could be that Karsus's Fall arc was not really canon; until it appeared in the many 3e products, like the Grand History of the Realms.
So my question is: when did Karsus's Fall become a canon part of FR lore? The question is mainly about which edition of the game, but more detailed data, or some sort of in-game information are also welcome.