Blizzard has a “it was only a setback” rule with an “Onyxia clause”. Unless we beat a villain twice, they are still alive. (Deathwing is technically getting a rematch with us in Aberrus, sorry, I mean Neltharion)
So no, I don’t think N’zoth or any of the old gods are dead, dead. And we will likely fight them again.
An Old God known for plotting, planning, and patience goes full “ME SMASH!” as soon as he gets out of jail. It’s a little too blatant, like he was putting a spot light on himself on purpose. Guess what if it’s believed Azeroth is cleansed of Old Gods no one is going to be looking for them and meddling in their plans.
You are all slumbering, dreaming, but He will awaken you to the true path. 
Ragnaros had to be defeated 3 times. Molten Core, Hyjal quest ending and finally Firelands heroic. And even then, he almost came back.
People just love throwing the word retcon around any chance they get. Can’t get enough! Mmm, retcon.
N’zoth tells us who are his devoted chosen. He gifts us his eye. He then at the end of BFA continues to whisper to his followers who have the gift after he was supposedly dead. Alleria Windrunner says, “I still hear him, and now his voice is joined by others.” Something like that.
If you walked around Stormwind/Ogrimmar at the end of BFA his followers who still had the eye can talk and get whispers from other NPCS telling you, “He’s not dead.”
He even tells us that his defeats are often his greatest victories because he strategizes every event and coordinates them to his favor. So even a devastating loss is an inconvenience for the old god.
Now in DF we travel back in time and he knows us. We basically provide him the path to avoid his own death.
Because it was sad that we blew him up in a single patch after years and years of build up.