Correct me if I’m wrong on this one, but I’m struggling to remember how he ever escaped Azshara and wasn’t fighting for her this expansion.
He got captured by Azshara and her kraken, followed by us beating the kraken, which then recaptured him and swam away with him again. How did he escape? I can’t really see a follow up.
Neptulon was already free in Legion, and part of the Shaman Order Hall stuff. They just cut the explanation. Originally there was a Vash’ir raid planned that would have resolved that story, but that got cut, or so I heard.
To quote WoWpedia:
The likely reason Blizzard had Neptulon end up freeing himself off-screen between Cataclysm and Legion is because the Abyssal Maw raid, the planned conclusion for the Vashj’ir storyline, was canceled. The situation was described by Chris Metzen as a “damn mess is what it was.” https://wow.gamepedia.com/Neptulon#Notes_and_trivia