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.