Parmandur
Book-Friend, he/him
Not really, there has been multiple edition support in detail for Cormyr and the Moonsea, as well as the Lands of Intrigue (Amn, Tethyr, Calimshan) and the Unapproachable East (Thay, Aglarond, and Rashemen). Even the Shining South has had at least two accessories in two different editions. Sembia was in a weird position at the start since it was supposed to be "left blank" by the developers for players to use as a blank slate, but that got dropped and there was decent coverage afterwards.
It's really the lands along the southern shore of the Sea of Fallen Stars from Turmish through Mulhorand that have really had lack of coverage, outsise of the pretty short module-length paperback accessories back on 2e. There a couple of other areas here and there that also lack sustained coverage (the Bloodstone Lands come to mind), but the area I mentioned is the largest and most notable.
Those areas got something, but still pretty light and with massive room for expansion by the DM compared to the Sword Coast.