This is also why I suspect people are generally nicer and happier on nostr, there is much less fighting because there is no algorithm that boosts angry and controversial threads.
not to say algorithms can't happen on nostr, there just aren't many in clients yet.
Nostr and services like it have a place but they are not going to replace services that have meaningful anti-spam.
Talk to anyone who tried running their own private mailserver. SMTP is a decentralized open and very simple protocol - yet that doesn't address the issues that keep people away from hosting their own mailservers.
My SpamAssasin 10 years ago would flag too many legitimate emails... while allowing genuine spam. The amount of crap I would get to my personal email was so bad, that I just switched to GMail for the simplicity.
It's just not worth the effort. Like it's just not worth the effort in investing into a decentralized social network at this point.
Maybe if the spam filtering protocols were public, trainable on all of the data(which isn't going to happen on a privacy focused network, ever), objectively efficient at catching spam... and runnable on even RaspberryPi 3 - you're just going to get spammed out of existence.