Now that I think about it, it almost appears like she was consumed by the forbidden spell. The obvious conclusion to draw is that she is dead. But I came to think of another alternative, was she perhaps also banished to the void together with Byleth? After all, she appears to be consumed by the very same darkness.
If so, this might be a worse fate than death, but she technically wouldn't be dead. Just stuck in a realm of nothingness.
What would happen to those that are stuck in that realm? Would they eventually starve to death or are they doomed to roam that realm for all eternity? Solon does call it a fate worse than death.