@Gargron @kai @jb55 Thank you for commenting!

The relays are not supposed to work without signup, it's only that the signups are out of the protocol. Relays can have a signup page and you paste your pubkey. Some can ask for email, others for money and so on. They can also filter by interests etc.

Then each user has to decide their relay-reading policy: it's safe to fetch notes from someone you follow from any relay, but you might only want note-replies from relays with good spam management.