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there's not been anything official said about whether Legacy is void or not, but it is somewhat suggested by the ending of Invincible.
Jag has left the Empire as a democracy, not a heriditary dictatorship, under the command of someone else. There's no real suggestion where the Imperial Knights may have come from (Tahiri being the "Emperor's Hand" seems to have been temporary and then dissolved to my reading). The GA is still goverened by a single CoS rather than a Triumverent. The Jedi are in hiding on Shadu Mau rather than Ossus and Coruscant (as shown at the beginning of Legacy).
Now, there's still about 92 years (43ABY - 135 ABY) for things to adjust themselves to match Legacy, but I like the suggestion that someone else on these boards made that "Legacy" will be the vision that Jacen turned to the dark side to prevent.
It actually makes sense with both his visions - the dark man (Krayt) on the throne (though I'm not sure how Allana beside him fits that), and a war that would consume the galaxy and cost millions of lives (All the jedi, and how many different planets were destroyed?) that that was what he was preventing.
And given that Luke is currently aggressively searching for the Mortis Artifact, and presumably shortly thereafter for Krayt (though he doesn't know who that is yet), I can't see the One Sith remaining hidden for almost a century. Even with the Kesh Sith to distract him for the time being.
Also keep in mind that Ship is taking Vestara to the One Sith on Korriban. There is no possible way that Ben, Luke and numerous others aren't going to hunt her in particular. There's no way that she'll quietly be written out and disappear.
Also keep in mind that Lumiya, Vergere, Kell (from Crosscurrent), and presumably others are regularly coming and going from Korriban either working with, or for, or even just briefly interacting with the One Sith. Sooner or later someone's going to get caught by the ejdi and say something that will at least cause the Jedi to search there.
I can't see any possible way for Legacy to occur given how FOTJ ended. And honestly, though Legacy was an amazing series, I'd rather that the writers not be tied to that.