Is ONE PIECE episode 461 filler?
Canon No, episode 461 of ONE PIECE is canon.
#461
The Beginning of the War! Ace and Whitebeard's Past!
決戦の幕開け！エースと白ひげの過去
Aired 8 Aug 2010
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.