Apple historically has not shown much interest in integrating direct support for third-party services, even in spaces they aren't competing in. In the few instances it has, these integrations saw little maintenance and were eventually removed. This makes some amount of sense with how rocky third-party APIs can be[1][2]. Generally, it's easier for both ends when service providers write their own integration for a given platform.

As an aside, the timeline given by Spotify is somewhat disingenuous because it doesn't include any of the times that APIs were opened up and is missing significant details about why rejections happened — for instance, early watchOS rejections (as far as I recall) had to do with the lack of a power efficient streaming API in watchOS. Once Apple had a streaming solution that was fit for public consumption (internal/private APIs almost always aren't and cleaning them up takes time), they added it to watchOS.

Don't get me wrong, I think iOS would be better if it opened up a little, but Spotify is no saint and has a tendency to embellish the story.

1: https://jod.al/2016/02/18/guide-to-poor-api-management/

2: https://www.theverge.com/circuitbreaker/2018/2/6/16979102/sp...