A record label did a test recently to see what happens if they make music available on Apple Music, then remove it. Record labels can choose to allow some or all of their music to be streamed, on an album-by-album or track-by-track basis. In this case, the label made an album available for streaming, then removed one track. When I try to download that track, here’s what I see:
However, I already had a copy of the track, and it still plays. My guess is that it will eventually not be playable, after iTunes has updated my iCloud Music Library.