I mean, relay syncing can act like any other client. Relays could just double as clients and listen for events on different relays.
Having a dedicated client do this is pointless, because Nostr clients are supposed to be connecting to (and posting to) multiple relays, perhaps a dozen or more. Clients should be notifying each other as to which relays they will be posting to, so that those who want to receive their messages can reliably find them.
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