I would be a bit wary of writing that for them, TBH. Like, I don't think it would be a dick move or anything, but I also don't think there's necessarily any easy way to predict whether they just don't want any additional main characters to die but would be cool with a fic focused on grief, or they mean it's okay to mention or imply that canon death still happened but aren't necessarily considering the possibility of a fic heavily focused on it, or they actually would rather not have any death at all but thought it would be a bit unreasonable to DNW the canonical death of a character who's been dead the whole time. Not that you can be expected to read their mind if they didn't give any other details, but it's still an area where I'd tread a bit carefully if they didn't explicitly opt in to it.