It’s been known for a long time. It’s implied with Krasus’s old home in the abandoned elven ruins, it’s been a bunch of easter eggs for a couple expansions now, a temple related to it was used for three class questlines in Legion
The highborne decided they’d rather move as far north as possible from it because it was driving people insane.
But based on the EK travelogue it seems like Shaw is surprised in his visit of Tirisfal to find inscriptions that are, frankly, bizarre. So, in theory, how much would you say things like these are common knowledge? I assume some people might feel it, and the elves might be aware that there’s a reason Quel’thalas 1.0 was abandoned after a few years.
But in general?