Evil druids go back to 3e (2000) in D&D. That was when the old 2e rule of them being True Neutral changed to allow Neutral Evil, Chaotic Neutral and so on. In D&D gaming terms, therefore, 'Shadow Druids' or 'Dark Druids' are nothing new. Their precise titles and motivations, of course, change according to the writer and/or GM.
Whilst I haven't read the old 2e Complete Druid's Handbook, I know that it contained a selection of druid templates. If anyone has access to that book, perhaps they might find a proto-Shadow Druid template therein?
I meant the specific type of druid called "Shadow Druid" that is tied to that druid from BG2, most of the Shadow Druid lore and indeed the order where the Shadow Druids in BG3 originate from ties into the games and not any older FR lore.