I don't know about Youtube Music, but Google Play Music had no problems in deleting entries from your own playlists when a song wasn't available anymore. It's a form of musical gaslighting which I find entirely unacceptable. We have emotional ties to some music, so editing my own handmade playlists without my consent, and leaving no trace of the activity feels like a violation. That single behavior has stopped me from trusting Google with anything ever again.

Spotify leaves the songs there even if they are unplayable, but my playlists are still there.

Spotify's recommendations were really good at the beginning. But now it's just meh. I haven't found anything interesting in Discover Weekly for months.

That wasn't in my initial plan, but I'm considering adding user recommendations to https://volt.fm. Something like Twitter, but for sharing music with your friends.

In case you’re still looking for a better recommendation system, I use https://last.fm/, I’ve found its recommendations the best. In other respects it’s a bit annoying (it plays music via Spotify and that connection doesn’t always work).