In both the Strixhaven: A Curriculum of Chaos D&D book and the Strixhaven Magic: the Gathering card flavour texts, forbidden magic is referenced. However, I can’t seem to find what is actually forbidden.
They seem to be fine with necromancy, as Lorehold college summons spirits, and cards like Moldering Karok feature undead living on or near Strixhaven campus.
The only things I was able to find was the flavour texts of a few cards:
Strixhaven forbade all magic from the blood age…
(Plumb the Forbidden)
This must not become common knowledge. The consequences of such a spell are too great. Lock it away in the restricted archive, and destroy its record in the catalog.
(Day of Judgement)
I can’t find anywhere that it specifies what magic was practiced in the blood age, however.