It's a reverse-engineered version of the "Spotify Connect" protocol. [1] The original name of "librespot" was "spotify-connect", as this fork shows.

[1]: https://github.com/Fornoth/spotify-connect/blob/master/READM...

As far as I know, librespot is reverse-engineered from libspotify and/or the official Spotify client. As mentioned on their GitHub page:

> Using this code to connect to Spotify's API is probably forbidden by them. Use at your own risk.