I think the Broken Isle have been common knowledge, they were even mentioned in warcraft 2.
From what we know though is that most of it was submerged, I can guess that it just wasnt a place worth visiting for those on Azeroth. So most like the Tauren and the Shal'dorei (or nightborne) there went undiscovered because of it.
I am sure there are a lot of islands out there around the maelstrom people are aware of but just don't travel there.