The problem with Twitter is that unlike RSS, it shows content from people you did not subscribe to (e.g. you get replies, you see strangers' replies to your friend's posts). The content is public, so unlike e-mail, receiving abuse is not just your private problem, but a problem for the network's image and reputation.
But in either case, when normal users (not righteous keyboard warriors) find some content to be horrible, they don't want to see it. If they keep being bombarded with unwanted content, they leave. For Mastodon/Twitter/Gab that may be "political" content, but the problem in general is similar to spam.
Can you point to the part on the SMTP RFC that specifies the spam deletion behaviour?
> XMPP servers will kick out spammers and harassers
Nothing prevents nostr servers from kicking out based on account or IP address or message content
> The problem with Twitter is that unlike RSS, it shows content from people you did not subscribe to.
This is not Twitter, any client can decide which content to show. And there are clients that do not show any content from people you don't explicitly follow.
It could be a community run closed system where only citizens are given access to the system.
It doesn't (and I think shouldn't) have any crypto features built in. That's up to the client builders to handle.
There's no algorithm that will curate your feed for you, it's up to you to choose who you follow.
Don't like what someone is posting? Unsubscribe.