We all know that promises in Haikyuu play an important role in the whole plot and the last promise fulfilled was Oikawa defeating everyone (or at least come back to play against everyone since it is unclear who won the match).

But i think we need to point out which promise is the most important of them all, and is the reason why everything happened in the series: Kagehina's promise to each other in chapter 37, which has 2 parts: The national stage and world stage.

The first that came into fruition is the national stage, as we all know, is their MSBY vs. Schweiden Adlers match.

The second one that we thought is the world stage is the Olympics. The Olympics is considered the world stage of athletes out there as we know, so this would be also considered as the world stage.

But let's not forget that this promise (in chapter 37) is originally not a promise of them playing together, but against each other, and that has been made clear since the beginning.