“They do not die; they do not live. They are outside the cycle.”
C'Thun in pre-WotLK comic plotted to have Cho'gall resurrect him and he even transformed him into the monstrosity he was in Cataclysm.
Yogg-Saron in Legion was still heard in Ulduar through his whispers and Brann Bronzebeard in Exploring Northrend (after SL timeline-wise) reported that the whispers of Yogg-Saron are still heard throughout Northrend and Ulduar in places where he once held power (such as Whisper Gulch in Howling Fjord).
N'Zoth in DF spoke to the player from the past and peered into their future, becoming aware of his own fate.
At the end of BfA, after N'Zoth's "defeat", Magni claimed that Azeroth was "freed from the grasp of the Old Gods"; but, the "King of Diamonds has been made a pawn."
We have confirmation that 3/4 of the Old Gods are still active in some form.
The Old Gods are outside the normal cycle. Death does not apply to them; they will always haunt Azeroth.