Relays don't moderate spam though. That's a generalization. Many of us have tried. But if the user enters just 1 relay that doesn't, then all your effort was wasted. So relay-side filtering isn't a reality in the present version of Nostr. The other option is client-side filtering. But then a spam attack, even if you can't see it, will effectively DoS the client because the client is still forced to download it all. You need a whitelist so you can collect only the information you want. Any filter without an "authors" property is doomed.