I was looking this up, and either there's going to be a second cour or a movie(s):
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So this is from MAPPA and the sound director, where you can see 'Final Season: Part 1' listed by the keyboard. He tweeted the image out and then deleted it. Unless its a photoshop or a troll.
Netflix Japan also said it was the final season part 1. And I know its easy to fake tweets, but I really do not give enough of a shit to go looking:
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I think its honestly going to be a surprise movie or movies. The series is ending, so a good way to bolster that revenue is do movies. I'm sure they'd get millions since as you said the Japs have shit for taste. But I will love the Western salt from that, because waiting for movies to get released in the states takes forever. 'Surprise Moviefilm(s)' makes sense to me. More cash, better animation budget, more revenue for everyone. It also makes more sense because why not announce a second cour when the season was announced? Plenty of popular anime, including AoT, have done this. So why not just say its another cour? Because they want the surprise of a movie drop. That's it really. So that's my guess. If its just another cour, I think they're stupid for hiding it because it'd have to be obvious to anyone you'd need more than 16 episodes to cover the last bit of AoT. I just don't see the point, honestly. Its not really building hype and people would be expecting it. But they wouldn't be expecting a movie.
Anyway, barring a second cour or movie, the AoT anime ends at chapter 116.