On Nostr, for a public key that I subscribe to, how can I verify that my relays aren’t filtering out messages from that key? Is there a merkle hash on your latest note that I can use to verify that...
You ask their relays for their content, not your relays. Relays don't have to keep everything from everyone and can't have what they don't know about. If your followers don't post to your relays, then your relays won't have it. That's why nostr uses the outbox model, so you know where to find your followers posts.
1