https://bluearchive.fandom.com/wiki/Yume#Trivia

She is the only student in the game that has been confirmed to be deceased, as well as the first character to be confirmed as deceased.

No ideas if she would be somehow sent to the current timeline in the future, though.

Do they resort to timelines in their story?

I couldn't persevere. Blue Archive story is a minefield. First very big try for me, it was the Juri's affection stories. "How could it make the cut?" was the only, painful, reaction I got, it was a quick downhill from there.
Second try I knew what to expect in the start of the Cherino's event or in the gaming club campaign chapter, so I got a bit further. But it's a total dump. The 3 year old level of storywriting. I've amassed thirty unread affection bits, again. Got hit by weekly Boss bits, which are always dumb pseudo-religious shorts. Club snippets I forbid myself from risking to touch. They've long since slapped on a cringe secretary as if the cringe shopkeeper was not bad enough already. After I read halfway into the third campaign chapter, where Eris goes everywhere around the campus, I was out of next chances to give this game.
The game has a ton of great music, while still being a very terrible production work in all the other aspects.
I can't believe, I don't want to believe, that the greatness of the story is what drives the artists to draw and uploaders to gather the Blue Archive stuff here.