Comment 46 for bug 938180

Any integration of Mixxx with Spotify would explicitly violate their API terms of use even if recording was artificially disabled. From https://beta.developer.spotify.com/terms/#iv :
"Mixing, overlapping and re-mixing. You may not, and you may not permit any device or system used in connection with the Spotify Service to, segue, mix, re-mix, or overlap any Spotify Content with any other audio content (including other Spotify Content)"

Rebroadcasting music from Spotify is also forbidden:
"Integration with Third Party Services. You will not create any product or service by integrating the Spotify Platform, Spotify Service, or Spotify Content with (i) any non-interactive internet webcasting service or (ii) streams from another service."

Related:
http://djtechtools.com/2017/09/14/spotify-axes-dj-software-streaming-virtual-dj/