So a group of us are trying to get a West Marches style campaign going, set in the eastern parts of Faerun. Maps of the current make up of the continent are pretty lacking. As I read around more, it seems more and more like WotC have retconned most of the Spellplague. So, would we be better off using maps from 3.5? Is that basically what is going on in 5e?
A bit late here, but the answer is both yes and no.
Lorewise, nothing was retconned, but there was quite a bit of reverting. So, everything in 4e still happened, and left scars around, but much of the world of Faerun did revert back to what it was like pre-4e. That said, there are areas that remained behind, most notably the dragonborn kingdom Tymanther has arrived in East Faerun, and the political landscape is pretty different. You should be able to get away with a 3rd edition map with some of the changes penciled in.