Obviously Belos just wanted to keep that sort of magic to himself, and telling people not to do certain kind of magic is kinda the purpose of the Coven System.
But joking aside, while grimwalkers and undead wizards fit the pop culture perception of necromancy, I'm not convinced that the people of the Boiling Isles would call those things "necromancy".
The one time the show confirms that necromancy is a known thing* is when Gus considers resorting to "forbidden sources" to do his interview of someone "interesting, accomplished and noteworthy". And in this context, the older definition simply fits better:
Necromancy is a form of divination that involves communing with the dead and/or summoning their spirits.
Which is perfect for what Gus wants to do with it.
And under that definition, necromancy would be part of the Oracle track. If it isn't straight up illegal, that is.
(* They don't use the word "necromancy", which might be because of Disney, but "raising the dead" is a pretty clear stand in.)