Spotify

You can pair any Bluetooth earbuds or headphones to the Apple Watch. But you won’t be able to stream Spotify. They haven’t brought any streaming functionality to their app.

https://www.wareable.com/apple/how-to-use-spotify-on-apple-watch-6609

https://www.ondesoft.com/spotify-converter-for-mac/play-spotify-music-on-apple-watch-without-iphone.html 

As mentioned, Spotify does not offer LTE streaming, even though watchOS 6 allows 3rd party streaming, as Pandora does offer LTE screaming, as well as Apple Music. For some reason Spotify is not interested in creating a steaming Apple Watch app.

Since Spotify is not an Apple product, you will have to talk to Spotify about their future plans.