My understanding is that the content is essentially self-hosted, so content removed from relays still exists on the posting user’s client

The relays are the hosts, not the clients. The only data the user carries is their unique public-cryptographic key to identify themselves & sign their messages. It's independent of client or relay, so they can use any at will.

Curation/moderation doesn't require relays. It can be done via the proposals/features mentioned before, which is distributed independent of relay.