Keydex is going to be the first Nostr app I'm aware of that uses relays exclusively to relay data from one peer's device to another, not for long-term data storage. I'm going to use NIP-40 expiration tags on all events so that they only live on the relay for a few days, which makes Keydex closer to a peer-to-peer application that uses Nostr as the transport (and identity) layer.