Is ONE PIECE episode 791 filler?
Canon No, episode 791 of ONE PIECE is canon.
#791
A Mysterious Forest Full of Candies! Luffy vs. Luffy?!
お菓子な森 ルフィVSルフィ!?
Aired 4 Jun 2017
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.