This isn't accurate. All of the relays do some filtering, so the algorithm is just a level lower. You don't know what was sorted out before it got to you, even if it was a bot or spam from one of the npubs you follow. The relays would eventually collapse under notes, if they didn't curate, at least a bit.
That is a simple attack vector: just flood them with nonsense notes saying "Happy Birthday!" or containing kiddie-porn, or whatever, and shut down the server.
Indeed, you can't prevent filtering, although you can improve the chances by subscribing to multiple relays, especially relays that you have in common with your follows.
But is there a nostr client that mixes in crap you didn't ask for? I think that's the main problem with "the algorithm". I'm amazed at how much outrage p*rn gets shoved down your throat in Twitter/X, Youtube, etc.
The closest we have to that is Primal, but you can get around their regarded Highlights feed, so it's not a real barrier.
