Our characters haven't even interacted with her directly in over a year, as liked as she is she's still a side character. Only way an on screen death would make sense is if our guardian was there to see it ( doubt osiris's pride would let him explain fully why and how to validate a flashback) and if we were there it woulda likely been a stretch for her to have to sacrifice herself. Xzivu can suppress light but probably cant stop the darkness most of us have on tap.