This is all I'm going to say. Rethinking about it, it's so obvious that N'Zoth is still alive. N'Zoth is a master manipulator and an illusion genius. He would NEVER confront his enemies directly in a brawl fight, he's too smart and conniving for that. He's smart and has been planning his return for eons and eons, so he must have developed layers upon layers of contingencies and backup plans in case something happened to him.
There's also the fact that the fight happened in Ny'alotha, which is just an alternate dimension showing a future where the Old God is successful. Alleria went mad in Ny'alotha but is still sane on Azeroth. Thrall went mad in Ny'alotha but is still sane on Azeroth. N'Zoth was destroyed in Ny'alotha... you guess how to end this sentence.
Or if Blizzard doesn't want to use the alternate dimension justification, they can just say that a fraction of N'Zoth's being transferred into the empty Blade of the Fallen Empire upon impact, providing N'Zoth with a backup plan in case his body was destroyed. Then within the Blade he can reform his essence and body and return.
Either way, it is clear that N'Zoth was not defeated in patch 8.3, he is still around and will most likely usher in the inevitable Void expansion, where reality itself will unravel.
A lot of people in the past complained about how "easy" it was to defeat N'Zoth, how "underwhelming" he turned out to be, but that was precisely the point, it was all intentional. You're meant to find it weird how a master manipulator could be defeated so easily, and you're meant to be lured into a false sense of security. That's what happened to the denizens of Azeroth and also to the fanbase of World of Warcraft. It is all planned.
N'Zoth is incredibly smart, to the point that defeat often works in his favour. He's not like the other Old Gods. He might be weaker, but he's far smarter and more cautious. This isn't the last we've seen of him, mark my words.It was here in ages past that the God of the Deep lost a great battle to the God of Seven Heads. But as was so often the case even defeat ultimately worked in N'zoth's favor.
In fact N'Zoth is probably grateful to the foolish Jailer. Grateful, because he's luring the pesky heroes away from Azeroth, making his job much easier. Thankfully the Ren'dorei and Wrathion have elected to stay behind, so at least the world has some degree of protection against N'Zoth's return.