Kirk McElhearn writing for Macworld:
But things get complicated when music that you have added to your iCloud Music Library from Apple Music is pulled. Labels can withdraw the right to stream certain songs and albums at any time, but you won’t be notified. You may see albums and songs in your library, but their titles are a slightly lighter color (depending on the view), and their iCloud status is No Longer Available.
It’s understandable that record labels make this decision at times. They may realize that enabling streaming for a certain album leads to lower sales, and the lost revenue isn’t compensated by the meager moneys they get from streaming. But it’s frustrating as a user to find that, for example, one or two songs of an album you’ve been listening to for a while are no longer available to stream. Or that an entire album is missing from your library.
Yet another terrific reason to use streaming services as a complement to — rather than a replacement for — a local music library. Unlike Spotify, however, saving playlists and music on Apple Music to your local library requires an iCloud Music Library subscription, which tends to cause problems like this. Conceptually, I still prefer the idea of one library that contains both, but Apple’s implementation of iCloud Music Library has been a widespread disaster. I’m not one for hyperbole or exaggeration, but there are few things more personal than my music library, and I don’t trust it to survive an iCloud transition unscathed.