I was debating between putting this in the raids forum or putting this in general, but as it's not specifically a question about raid content and this forum is read more frequently, I figured it'd be better put here.
I'm putting together a massive soloing guide, and I'm trying to make it as comprehensive as possible, so that even someone who doesn't have any idea where to go or what to do can follow along. In addition, part of said guide is putting in a lot of historical information about old raids, and part of THAT is their release dates. I'm currently working on the BC section of the guide, and one raid in particular is flummoxing me with regards to when it was released: Hyjal.
Wowpedia says it was added at BC's launch, though I seem to recall reading somewhere (I can't remember where for the life of me) that it wasn't opened until patch 2.1. I've yet to find a concrete source on this, so I turn to those who were raiding/playing back then to help me!
They didn't even bother testing Hyjal until the BT PTR patch.
The official word is that Hyjal was in game at launch, but that has never been proven. Nobody was ever able to zone into Hyjal until the BT patch due to tuning issues with Vashj and bug issues with Kael.