The lower half of the main island
was originally going to be part of the base game but it ended up being cut. tbqh the old stuff is the only reason I'm excited for Summerset--continuation of the Triad arc aside--even if the art direction for the main island is a massive disappointment.
(There's a good chance NONE of it will come back, too, which is worse IMO. There seem to be some hints of it in what little we saw of Artaeum, so there's that.)