I don't think that Nayuta has actually died because she haven't fulfilled her purpose as a character, we already saw that her motivation to Denji happy were quite malicious but I think they have changed over the course of time, now she genuinely wants him to be happy with his current family, I mean, why else would she be that hurt when Denji told her to stay away? I think it would be quite a turning point for Nayuta, since she have lost her only identity, that is being Denji's family.