It does indeed happen. There seems to be a few different types of it. Sometimes a whole band will just disappear off the platform - chalk it up to contractual disagreement. Sometimes a particular album will not be available on Spotify, but other albums from the same band will be - likely some issue with the label that produced the album. Finally, sometimes the particular version of the song you like (e.g. a certain live version, acoustic version, whatever) will disappear but others will remain (e.g. studio version).

However, it really doesn't happen that often. I probably have a few thousand songs save on Spotify and perhaps a dozen have been taken off after many years of using the platform.

Spotify has left me doubting my sanity a couple of times by removing albums I could have sworn I had listened to before in the platform.

And I had, but they removed them since, listing nothing in their place.

Spotify does this too, at least in my region (Estonia). It's frustrating, but not that frustrating to go back to downloading MP3.

It happens, but pretty rarely in my experience, though I'm probably not the most avid consumer of music so other people might have a better grasp of the extent of this issue on Spotify.

I've had it happen on both Apple Music and Spotify. Given my 40h+ master playlist, I tend to notice it. I always have a few gray holes.

As for replacements, my understanding is that they are often due to the better royalties on original artist re-recordings. I always try to switch back to the originals, and AM, my current service, only sometimes still them. Frustrating to build a collection on shifting sand.