How did she die in this film, but was alive in the original X-Men films? Sorry if I'm missing something.
Continuity hasn't mattered in these movies for a long time.
Oops! I don't think we were supposed to notice that.
The events of Days of Future Past caused the timeline to diverge so the X-Men films with Patrick Stewart et al are of a separate timeline now.
Considering they showed this in the trailer, I wouldn't consider it a spoiler.
Most people didn’t see the trailer.