According to the voice of Astarion himself, there's a whole two hour section of Baldur's Gate 3 no one has played yet.
"There's even something I know about that you can't get to unless you do something that I don't think anyone's going to work out," Newborn said in the livestream, which you can see him talk about in a clip above. "I was told this in confidence and I think I'm one of the few people who knows about it as well." In the stream Newborn was even asked if he could tell the person he was talking to what it is, but noted "it's one of those few things I cannot friend-DA." Just to clarify, Newborn will have signed an NDA to work on this project, unsurprisingly, but people are people and they do tell their friends things, but this seems like something so secretive he can't even do that.
Update: See below, it has been found.