Alignment still exists but it isn't as mechanically significant. The list of mechanics and abilities that care about alignment is fairly short, and the list of creatures that care is shorter.
But this isn't about mechanics. It's about lore. It's about story.
I don't really care that much about mechanics anyway. I've already stated how I very much dislike D&D mechanics and consider much of it to be broken or silly. WotC removing alignment from mechanics is just another example of D&D mechanics sucking.
What does matter to me, and matter greatly, is story and lore. That's pretty much the entire reason why I play cRPGs. And as a very longtime fan of Forgotten Realms lore, the consistency of that lore in BG3 matters to me. And lets also remember that Larian has repeatedly said they respect existing D&D and FR lore in making this game.
