Monsters, One Piece's planned prequel, is canon, following Wano's Shimotsuki Ryuma, who handed Zoro Shusui.
Monsters, the newest anime series on Netflix, has finally debuted, and it seems like the One Piece prequel might be considered canonical.
Based on Eiichiro Oda's one-shot manga of the same name, Monsters is almost certainly a prequel that follows One Piece's Ryuma with a few clever alterations.
Netflix's Monsters' post-credit scene establishes that Ryuma is the legendary Wano samurai who left Zoro with Shusui on Thriller Bark.
After their defeat, Ryuma's zombie (animated by Moria using Brook's shadow) throws Shusui to Zoro in One Piece episode #362, indicating they are one.
Monsters' anime version gives Ryuma Shusui instead of the manga's katana, thus legitimizing the prequel.
Shimotsuki Ryuma killed a dragon in Wano, not abroad, but their commonalities make up for this.
Ryuma's zombie's gray hair and Monsters' disappearing scar over his eye are also signs of age. Monsters frequently calls Ryuma a foreign fighter to remind viewers of Wano.
Oda stated in the SBS section of chapter #457 of One Piece that the mythical fighter of Wano was significantly influenced by Monsters.
A canon prequel is likely from the one-shot. Calling Monsters a prequel may have benefited on One Piece's recent success and Netflix's One Piece monopoly.