i think by now we already know that is just a myth/lie from the devs, at least, not the way you are thinking.
They have the overall aproach and theme of the expansion, what happens there isn't set in stone and things change all the time between patches and even the end can change. Like characters whow ere not supposed to be relevant or appear at all became relevant because the fans liked.
Sylvanas portrayal was straight up something changed for the expansionw, and it was adapted to fit it, the proff is how she was build with a common sense of continuinty since wtlk, including the books that pre-date shadowlands, but then, it was everything changed.
Legion had all the necessary "nuances" to show why Sylvanus went full loco in BfA, she fucked up her deal with heylia, her valkyrs were dying out, she wanted power and to conquer easter kingdoms, she even said she aimed for stormwind, conquering the humans and bend then to her as shields against mortality made 100% sense with the character, and tacking account how undeads can go wild like that.It's way too easy to just blame Danuser when it was BfA the one that hard left with Sylvanas, who up until Legion had that big question mark. That's the huge red flag with BfA, it removed all the nuance necessary to carry Sylvanas from Legion to Shadowlands, cause, again, by BfA they already knew what the next expansion the other team was working on.
The problem isn't that BfA made her a villain, the problem is that BfA made her a villain without consideration of the story the next expansion was going to tell.
Or, you know, the most logical thing, it was how things were uspposed to be, she being villain and still being a villain in shadowlands, and someone else take the spot of protagonist, like bolvar, the players, or anything else, but Danuser assumed and decided to gave a enw direction to Sylvanus, cause her portray right now goes against the entire trajectory of the character, from cataclysm to Legion, enven if you excluse BfA anyway.