Spotify wants all the world’s music on Spotify. However, some artists and tracks aren't currently available. Sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
However, please know thousands of albums and artists are added to Spotify every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon.
In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.
Taylor Swift left because she wasn't satisfied with Spotify's service. More exactly that both free and premium users can access the music library with same rights: "It's my opinion that music should not be free, and my prediction is that individual artists and their labels will someday decide what an album's price point is.”
It's likely that her music will someday return as Universal Music Group and Spotify had apparently signed an agreement that allows only Premium users to listen to the new releases of Universal Music Group artists for the first two weeks after releasing.
Long story short: Spotify is very happy to receive and upload all music they get, but artists have the control over what music they'll give to Spotify and how.
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