Im caught up on the anime through the Elbaph arc and keep seeing people bring up Film Red and Strong World like they actually matter to the story. But I thought basically all the One Piece movies were non-canon side stories. Is there an actual answer on which ones connect to the main story, or is it more of a case-by-case thing?
it's genuinely case by case, you had it right. the blanket "movies are non-canon" thing is mostly true for the actual plots but Oda has a habit of quietly promoting specific details into canon after the fact even when the surrounding story stays fanfic-tier.
Strong World is the clean example: literally none of the plot (Shiki's plan, the floating island, all of it) counts. But Shiki himself got folded into canon later — he shows up in the Rocks Pirates flashback stuff as the first guy to ever escape Impel Down. So the villain is real, the movie he starred in isn't.
Film Red is the same deal. Uta existing and being connected to Shanks is treated as real going forward, but the actual story of the movie (her plan, how everything plays out) isn't canon. Pretty sure some throwaway detail about Shanks in Red also ended up getting confirmed in the manga before the manga "officially" revealed it, which is very on-brand for how Oda uses these movies as a testing ground.
so: skip them for plot, but don't be shocked if a character or one-liner from a movie becomes real later. Strong World and Red specifically are worth watching for that reason even though you're not missing "story."
Short answer: mostly skippable, with two frequently-cited exceptions people bring up for a reason.
Neither film's actual plot events line up cleanly with the manga's geography or timeline though, so treat them as "Oda touched the character/idea, not the events." Everything else - Z, Gold, the older ones - is a pure side story with zero story-relevant carryover. If you're caught up through Elbaph you're not missing plot information by skipping any of them, you'd just be missing the two character ideas above, which people will happily spoil for you in a sentence like I just did.
