Soul Stone was located on the planet Vormir, guarded by the Red Skull. It wasn't until Avengers: Endgame that Marvel explained why the Soul Stone was there, with a subtle line of dialogue - but this explanation is contradicted by Eternals.
Marvel had been setting up the Celestials, Eternals' cosmic space gods, since 2014's Celestial setup in the MCU is contradicted by Eternals.
According to Eternals, the Celestials are the cosmic beings who created the universe itself. They are so vast in size that worlds are like incubators for their eggs, with a Celestial seed contained within a planet's core nourishing itself on the life forces of the planet's inhabitants. The Celestial seen in the Collector's recording is a midget in Celestial , and it certainly wouldn't make sense for their civilization to be centered upon any planet, simply because a single world couldn't really contain more than a couple of them. Even the connection with the Infinity Stones was ignored. All this means Avengers: Endgame's throwaway explanation of the Soul Stone's location no longer makes sense. Vormir was never the center of Celestial civilization, so the Soul Stone must have been there for a different reason.
Marvel Studios has a reputation for playing the long game, and for the most part they pull it off well. However, the studio is actually a lot more reactionary than many viewers believe, and it's not unusual for setup to be abandoned in favor of a different idea. The Avengers was clearly setting up a comic-book-accurate Thanos, for example, while the murals seen in Guardians of the Galaxy pointed towards an Infinity Stones plot featuring cosmic avatars. Yet, when Marvel finally settled on their portrayal of Thanos and the Snap, they abandoned all these ideas. Clearly something similar happened with the Celestials, with Marvel deciding to take things in a different direction and forget the setup.
Some viewers may well be irritated at these inconsistencies, but in truth they show that Marvel prefer to give their writers a degree of creative freedom to tell the best story they possibly can. Continuity is important in a shared universe like the MCU, but it is a useful tool - and a poor master. Marvel's approach to continuity is the right one, because the minor inconsistencies caused by their change in plans can be ignored by most audiences - and Eternals certainly took the right approach with its portrayal of the Celestials, resulting in a much more rewarding story.
