"Died off" is a colloquial phrase used to describe a decline or disappearance of something or someone. There are a number of synonyms that can be used in place of "died off" depending on the context. "Vanished" can be used when something or someone suddenly disappears, while "faded away" can describe a slow process of decline. "Extinct" is commonly used when referring to the loss of a species, and "perished" can be used when discussing death due to natural causes. "Disappeared" and "ceased to exist" are other options that can be used in different contexts. Using an appropriate synonym can add variety and flair to writing.