Potentially, or everyone but the dead are corrupted. She is clearly letting her Dark Rangers in on some details of what is going on, they speak of ‘What [she] warned us about’ in regards to their Shadowlands damnation - but how does she know they are all damned? Her growing affinity with death may indicate she knows something about the nature of the afterlife others don’t.
When she burns Teldrassil, her last thoughts before ordering it are that (paraphrased poorly) “She knew it would come to this, but just not this soon”, with the context that she suspects Saurfang’s hand may have been stayed by outside influence. Far from being a whim, the Burning was a recalculation to her belief that some larger game we’re not aware of was more advanced than she’d thought. Now, what ‘this’ is in this context, be that gathering a huge mass of the dead or striking at Old Gods/Elune/??? with the Burning, is unknown.
Her new dialogue is laced with references to larger plans that others aren’t aware of, and she seems to sincerely believe the Alliance are blind to some threat she knows of that isn’t her. She claims Anduin is deceiving them, which may suggest the ‘master’s table’ angle has more legs to it.
Why do I mention all of this? Because it may be that Sylvanas, being dead and knowing more about the afterlife, is immune to a pandemic corruption of the world. The world is bathed in Azerite now, which several old god comments suggest may not be what it appears (again, poorly quoted) “Its gleaming surface hides shadows beneath”, “the king of diamonds has been made a pawn”, “Her heart is a crater and we (old gods) have filled it”.
It may be that Sylvanas is brutally confronting the real battle for Azeroth and can’t tell us her plans because we are unwitting spies for the enemy.
And you may say ‘But surely the Forsaken PC, being guarded from the Old Gods by undeath, should be told!’ - and that is a very good point… but that same PC is holding the Heart of Azeroth, the nexus of corruption in this case. It may be that only her inner circle are informed of what is occurring.
This is all major tinfoil hat level conspiracy, but Blizzard is so all over the place I don’t think it’s totally off the table.
Fingers crossed!