Tabard of the Argent Dawn has been available at two times in WoW’s history.
First was during the original Scourge Invasion event just before the release of Naxxramas during vanilla (Patch 1.11 was June 2006)
You needed to collect Necrotic Runes, there was a short quest and then you could get the Tabard from the vendor.
The second time was again during a Scourge invasion, or Zombie Infestation as it became known that took place at the end of Burning Crusade as part of the pre-release events for Wrath of the Lich King. (Patch 3.0.2 was October 2008)
Again during that event you had to collect Necrotic Runes for a short quest and then buy the Tabard off a vendor.
So it’s been a really, and I mean really long time since this one was available and is definitely a Tabard with heritage for sure.
As much as I wish it could happen, I don’t think another Scourge Invasion is going to happen in future, could be wrong, would love to be wrong as it was a massively fun event at the time