Oda revealed that things are going to be grim for Sabo, Boa Hancock and Vivi - One Piece (opfanpage.com)
Now here's the caveat to the message:
- It was supposed to tease what happens the following year (2020) at the time
- It also mentions "when its all over, things will look grim"
So either, Oda entirely meant for Wano to end in 2020 (lol) or he's just implying that he will tease these things between Acts 3 and 4 about the things happening "after Wano ends". Translating Japanese is weird so we can't entirely be sure of the meaning.
Again though, Shanks is "supposed" to be moving this year (2021), based on Oda's Jump Festa 2020 message. So either we claim that Wano is ending in 2021 (which still seems unlikely but I guess possible) or again, there is a mid Act update. He could also show up this arc I guess as well but that seems a little ridiculous at this point in the arc.
Finally, it just comes down to the pure fact that 993 had an advertisement on its cover page that said Act 3 was ending soon. Yes. its been 10 chapters, but relatively speaking the Act is currently 45 chapters. If it ends at, lets say 50, saying this 35 chapters into the Act isn't a lie.