He doesn’t have to be outwardly referenced at all to be foreshadowed. The resurgence of the Cult of the Damned operating out to Scholomance is enough. The fact that they went to great lengths to steal the Book of Medivh, an artifact Kel’thuzad wanted, an artifact the Lich King wanted… That is enough to foreshadow the return of Kel’thuzad.
I’ve noticed something about you, just by reading your posts. You absorb lore on a very surface level, you see things as very black and white. You lack any imagination, you don’t see the bigger picture, you hyperfocus on small insignificant details, ignore anything that doesn’t fit into your tiny preception of reality. I’m not trying to make ad hominem, just pointing out how frustrating it is having a conversation about lore with you. It’s like talking someone to an art show, who only looks at the brochure in their hands, the painting is right there in front of you eyes, just look up!