Seriously guys, do you know what Immortal means?
It does not mean unkillable. Or Invincible.
Mortal=you die eventually. You decay.
Immortal=Not the above.
Sylvanas is not the above.
Sylvanas is immortal. It has nothing to do with wow standards.
She is NOT unkillable.
She is NOT invincible.
She is NOT indestructable.
She IS immortal.
If you don't believe me, maybe you will believe in Oxford (I will include their example sentence as it's very fitting)
1. Living forever; never dying or decaying.
‘our mortal bodies are inhabited by immortal souls’
If you leave Sylvanas on her own, she will survive forever. Just like the thousands of years old night elf banshees and ghosts we see here and there.