The NPC that’s supposed to be the only Alliance seller for this recipe IS still at Silverwind Refuge, but Silverwind Refuge is now Horde. And Harklan Moongrove is dead. So along with the Big Iron Fishing Pole only being available now with a Horde-only quest, there’s no Alliance seller or other way to get the Recipe: Shadow Protecton Potion recipe?
The only other recorded vendor is Christoph Jeffcoat, a Horde trader in Tauren Mill.