One Piece finally saw the final phase of the fight between Luffy and Katakuri in the Mirror World, and it capped off with an amazingly animated sequence in which Luffy debuted a brand new Gear Four form and Katakuri went all-out with strength of his own.
The anime's confirmed that it definitely was the end of the battle at last as the preview for the next episode of One Piece teases that a winner will be crowned "finally."
There admittedly seems to be a bit of meta-commentary in the title for Episode 871, intentional or not, as it's officially titled "Finally, It's Over! The Climax of the Intense Fight Against Katakuri!" The episode's title confirms that the battle between these two, which has been going on for several weeks now, is finally at its end, but there's no confirmation just yet of which one will be the victor in the end.
Though fans of the manga run of the series know who walks away, and fans of the anime can most likely guess as to who won, the worries for Luffy and the others will not be over in the slightest. Katakuri was only one of Luffy's major problems as fighting him in the Mirror World only served as a means to an end. It was meant to distract him from the other Straw Hats, who are still being cornered by the Charlotte Family.
The preview emphasizes this as it states that both Luffy and Katakuri have been completely drained of strength and the victor will be exiting the Mirror World to find a total Charlotte Family ambush waiting for them. Sanji is at Cacao Island waiting and hoping Luffy is the one to emerge from the mirror, but it's going to be a literal case of out of the frying pan and into the fire.
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has since been collected into over 80 volumes, and has been a critical and commercial success worldwide with many of the volumes breaking printing records in Japan. The manga has even set a Guinness World Record for the most copies published for the same comic book by a single author, and is the best-selling manga series worldwide with over 430 million copies sold. The series still ranked number one in manga sales in 2018, which surprised fans of major new entries.