What kind of backward logic is this? You do realize that all of Shadowlands will be about thwarting Sylvanas, right? Your toon will denounce Sylvanas eventually, whether you like it or not.
And actually your toon already denounced her. No matter what you do, you will ALWAYS receive a call from help from the new Horde Council, begging you to come to Orgrimmar and help them fight the Scourge, and that pop-up quest CANNOT be declined, it can only be accepted. So sorry, your toon already denounced Sylvanas and accepted a request from the new Horde Council. Sorry not sorry, really.