In a report from TechCruch, it appears that one of Apple’s WWDC developer sessions spoke about a new “Play Media” intent that is part of Siri Shortcuts. This would allow users to call up media such as audio and video from third-party apps, which effectively means you could use Siri to control music playback from other streaming services that aren’t Apple Music.
However the only drawback to this is that it requires developers of such apps to actually integrate it into their apps to begin with. This means that if developers choose not to do it, then we guess you’re out of luck. Plus chances are it won’t be quite as comprehensive or as powerful compared to controlling Apple Music.
In any case only time will tell if music apps will start to take advantage of this feature, so keep your fingers crossed if you’re hoping to see more Spotify functionality come to Siri!