I've never seen this work. Either it plays the stuff I've listened in the past in a loop or shove some random things I really dislike (maybe hidden promotional stuff?). Personally it's the reason I've cancelled subscriptions each time I've tried, I always ended up listening to radio instead as the value brought by Spotify etc... was really poor.

I use YouTube Music and it definitely does work but yeah it weighs songs you've already said you like way too heavily and generally seems way worse at discovering similar music than Last.fm or Pandora from over a decade ago. (If anything I remember Pandora being too good at finding similar songs - the playlist would end up almost monotonic because it found such similar music.)

Fortunately they do have a "Discover playlist" that completely excludes music you've heard before. Unfortunately that's all you get. No way to e.g. say "play me reggae I haven't heard before", and it's only updated once a week.

So yeah... kinda shit. But still better than the alternative which as far as I remember from the 90s is to only listen to extremely well-known bands and find good news music like once a year.