Yes it is completely banned. The Kirin Tor have outlawed it on pain of death. And a class fitting 100% outside of lore just shouldn't exist. With DKs and warlocks, they are tolerated as a necessary evil but DKs are constantly on thin ice. Warlocks have to only use fire and shadow magic to avoid execution. Necromancers have absolutely no abilities that aren't vile and evil. Their magic is all about the channeling of death magic and defiling bodies. DKs at least can avoid using necromancy by warping runic magic into a different type of power. Necromancers wouldn't have that luxury. Necromancers just wouldn't fit with the lore and if they were ever made into a class, it would show Blizzard doesn't give a damn about consistent story anymore in any shape or form.