I think noster deserves to be part of this conversation too. Instead of users creating an account on a server that can remove them if it wants to, they create a public and private key pair and publish their notes to several servers, all of whom have a copy of it and can distribute it further.
This adds redundancy because if one relay decides not to host your content anymore or goes offline, your content is still on like four or five other relays accessible to other people. And if for some reason every relay on planet Earth banned you, you could always run your own very easily.
Spam mitigation measures exist in the form of some relays requiring a proof of work that calculates the hash of the content with a certain number of leading zeros to prove that you spent energy to create that hash. There are also a few paid relays where you submit your public key to the relay and pay a small fee, and then that relay will host your content when you publish to it.