I wouldn't say unkillable. A small slash from Shalamayne scared her face pretty badly. Without plot armor that could have probably been her head. But we are getting off topic on something that we will probably see soon in Shadowlands. We will learn how resilient she is.
Jaina now protects her father instead of letting him die.
And two interesting scenarios. Thrall telling Garrosh both the good and the bad of Grom Hellscream and Garrosh is not the typical Hellscream but a diplomat who will defend himself if needed but only if needed and not a megalomaniac monster.
Sylvannas when gaining her free is the same as she was in life and actually cares for her people working with both factions to cure undeath instead of becoming the enemy of Life.