Board Thread:Boruto Discussions/@comment-27924662-20180920215259/@comment-1512913-20180920224005

Nope, she is not dead. Momoshiki actually got killed, Kaguya didn't, just depowered and sealed. I'm sure she can be killed too. She is only immortal in sense of not aging and not requiring sustenance to live. Had the Kamui managed to cut off her head, I'm almost positive she would have been a goner. So she is just ageless and semi-self sustaining, but can get destroyed, she is just extremely hard to destroy, so sealing her was a much better option.