Isn't this fixating a bit on the wrong issues though? It's not a matter of whether something should be familiar or not according to the general Lore, but as Niara and others stated an issue of the Player not knowing what their Character (and pretty much everyone else in the setting) is meant to know. It's a conveyance problem, not just for the Shar stuff, but for the whole religious lore backdrop and the Pantheon writ large.
It's the DMs job to deliver the lore in a way that the player can comprehend, for the player's benefit not the characters. If the DM is talking about minerals and symbols and the player is staring at them with a blank expression, the DM might set them aside and say "hey, in this playgroup you're expected to know a couple things about the game setting before you show up!" But it's a little different if you know for sure that your player is a first time player without a clue. In that case you'd probably preamble it and do some story-time off the cuff to bring the player up to speed. That should be Larian's starting point, with no assumption there, and some definite effort put into introductions.
Sure we could throw a lore book in the new player's face, but that's not terribly endearing or exciting. I think the point is that Onyx might be Shar's without dispute, and the onyx stone in the lockbox even says that pretty clearly, even before we could free Shadowheart from the pod, but a new person who is not in the know wouldn't know what to make of that. Or at least I thought that is what the OP was trying to communicate right? That more general frustration that the game isn't setting up the setting, but just expecting us to already be familiar with it. That's fine for half of us, but not for the other half.
Someone came in here raising a pretty legitimate issue with the presentation and exposition and how the game assumes too much prior knowledge on this core part of the lore, and within a page of comments, was then basically run out of their own thread for not having enough prior knowledge about the lore here too lol.
The game can do better, and it really should. Even BG1/2 didn't do a great job of this stuff, so it's not like we can look there for the best in show. D&D has a problem with this stuff in general, which is the thing that prevents it from gaining more mass appeal I think. I'm not sure really, but I think part of the experience when it was introduced has made them a little wary of trying to provide a D&D catechism. But that's nevertheless a big part of what makes Faerun, and the Forgotten Realms did pretty it up pretty explicitly. Sure it was guarded, and a bit in response to what happened on the first outing, but it was also well received by players who like Legends and Lore and Mythology type stuff. It was a strong hook for the setting I think. One that they're kind of letting go here, by allowing the Absolute to upstage everyone else in the Pantheon, and not doing any preamble really. I can imagine ways that they might introduce this stuff, or perhaps are planning to eventually, but it's not in there yet. Hence the critique
Do they need to do an intro movie before the prologue? Because we're talking about nearly 50 years of backstory. How much of it do they need to cover, because I could see that intro movie going into hundreds of hours trying to cover all the canon possibilities over that time frame.
The other side of this is, of course, the fact that this scenario will come up in every original IP anyone plays. Someone jumping into Dragon Age with Inquisition will be every bit as confused about story beats that involve the Warden, or Hawke. What about Kieran? Does he even exist? Who is he? How did he come to be? All stuff that anyone that played Origins and DA 2 would know, but a new player won't. What about Mass Effect, what story beats are going to be confusing to someone that starts the trilogy at 3? At some point, a player that wants background information, such as who Shar is, might rightfully be expected to do some footwork. Even in a TT setting, players would have access to the books.
