Is ONE PIECE episode 836 filler?
Canon No, episode 836 of ONE PIECE is canon.
#836
Mom's Secret! The Giant's Island Elbaph and a Little Monster!
マムの秘密 巨人の島と小さな怪物
Aired 13 May 2018
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.