I suppose character death warnings come down to being up to the individual writer. If someone's posted a fic rated R or NC-17 and classes it as 'angst' as far as genre goes, a character death warning is probably not necessary, since the reader knows they are reading something potentially upsetting. If, on the other hand, the fic is genre: 'genral' and PG-13, a character death is not really something the average reader would believe to be on the cards.