Yes. It is necrophilia if it's dead or undead.
Granted, there are some undead I'd hit. Vampires, incoporeal undead (ghosts, banshees, wraiths-- assuming you can find a sexy wraith, which I doubt-- and mummies if, and only if, she were able to reconstitute her body fully, NOT an illusion.
Basically anything that has/can/will rot, is a hard pass. Sylvanas is a hard pass. She's literally possessing a rotting corpse, folks.
I'd also not hit jiangshi-- the Chinese hopping vampire. Because, yknow, the hopping implies rigor mortis. So noooope.
Even the cute flesh golem girl here, is a nope.
Whiteleaf Studio, is it possible to avoid sex scenes with specific characters?