Can't clients enforce the moderation rules? For public groups, where everyone can see the posts, you don't actually need relay cooperation do you? A NIP-29 aware relay could restrict event delivery to group members, and provide some privacy, as a value add, though. I'm not sure how inclined I am to trust such a relay to never leak, and since nostr events are non-repudiable, all it takes is one mole to ruin privacy.