Is ONE PIECE episode 843 filler?
Canon No, episode 843 of ONE PIECE is canon.
#843
The Castle Collapses! The Straw Hat's Great Escape Begins!
巨城崩壞麦わら一味大脱走開始
Aired 1 Jul 2018
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.