Shar is like Satan in FR. She is well-known and feared and detested. And yes, maybe some would not know her symbols, but many would especially Gale and druids in the Grove.
Well, this is too easy. There are populations in Africa today that are cut off from modern society, some by choice some because they're next to impossible to get to, and they wouldn't have a clue. Stepping back 200ish years, the Native Americans had no idea. Christianity ran roughshod over most of Europe and parts of Asia, but always managed to find "heathens" to convert. Was God less immortal back then? It's not hard at all to imagine that some people wouldn't have an idea about who this God or that Goddess was if they hadn't been exposed to them at some point.
Okay, but the Sword Coast is a limited region in Toril, a region defined by the worship of these gods. So our analogy should be restricted to a similar Earth Area: let's say the east coast of the US where Baldur's Gate is Washington DC. Given that Tav is, by default, from Baldur's gate, the appropriate analogy would be someone living in DC not having heard of Satan. At the very worst, Tav might have spent his life in rural South Carolina or Arkansas, but should still know of Satan in those cases.
Your analogy would be someone living deep in the jungle of Chult having not heard of Shar, which sure that's fair. Although actually this analogy is too generous, because you specified "cut off from modern society" whereas Chult does have communications with the Sword Coast.
