A nation of the living dead is the sort of thing that makes headlines. Only truly isolated societies like the Pandaren seem to be oblivious to who the Forsaken are.
And there is a cute moment early in Pandaria where some random rice farmer assumes Shademaster Kiryn was injured severely in the zeppelin crash and is like on the verge of tears that she’s somehow still moving. Then your undead walks up and goes Ahem, no, that’s just what we look like.
The Sin’Dorei of all people had to be well aware of the Forsaken. Even if their ex Ranger General wasn’t leading them, they’re still neighbors. The Belves were busy not oblivious. Blizz didn’t have to show us them learning about them, it was safe to assume Farstriders were already keeping tabs.