My potter fanfic is on AO3, but similar debate. Voldemort...dies in canon, pretty typical for villains, so I didn’t worry about it there (Ditto for Dumbledore). However, my main characters do seemingly die for an extended period of time, and that’s what prompted me to simply add in MCD and be done with it. (And now that I’ve got Harry jumping into alternate realities where those alternates do die, it’s kinda warranted.) I’d rather err on the side of tagging things, than not.