There's nothing about nostr that prevents any relay and/or client from offering these things (content moderation, centralized discovery, spam filters). They could even share their findings with other relays/clients and build one big cohesive censorship system.

If this happened, users would be free to opt-out of that and switch out their client and/or relay with other options that fit their needs without friction. They would keep their identity, note history, friend list, etc.

Could be as simple as one client releasing a UI change the user hates. They could just download a different app and keep going.