The availability of music on Spotify is up to the artists and their music label.
If the music is gone, the artist or their music label has likely decided to remove it from Spotify.
Sometimes the music is only available in certain countries, while access is blocked to others.
Sometimes the album or song will be unavailable temporary, and come back again later, like they told you.
Hopefully it will come back later, but depends in the artist or their music label.
More info here:
https://community.spotify.com/t5/Content-Questions/I-want-an-artist-song-on-Spotify/td-p/1136005