Having just read through most of the new Shadovar wiki entry (excellent and thorough work, by the way), there's a contradiction I've found. I'm going to keep it brief.
In the section titled, What the Shadovar Commoner knows of their history, it states that, in essence, Karsus' Folly is unknown to them. Later, that it's a state-protected secret.
In the book titled, Into Darkness - The Birth of the Shadovar, it outlines the Folly. In the Deity book about Mystra, it outlines the Folly. In at least a half-dozen player-written books, the Folly is mentioned.
Considering the retroactive restriction of eldritch knowledge, and any Far Realms lore (up to and including their very existence) in the past, I'm concerned this is a sign of another rewriting of lore.
Or, it simply may be that Arelith has a fair few scholars, and in the Shadovar empire, they don't allow the discussion of Karsus. A commoner who comes to Arelith may discover this 'secret', but entirely dismiss it as folly in and of itself.