I already thought a lot about this while designing a gossip protocol for Indra. Not having expiry for ephemeral data makes managing the purging of caches complicated, much simpler to put a reasonable time window on it.

As far as this nostr protocol goes, quite frankly, it's amateur hour in protocol design, and the supposed designer of it amply demonstrates on a regular basis how little he understands about security, distributed systems or even the most basic things like why bitcoin doesn't benefit from being anchored to shitcoin chains.