To defeat the NIP-04 metadata leakage, just have everyone DM everyone constantly with throw-away messages. That is, content that undecrypts to identifiable garbage.
Saturate the graph with noise.
Discussion
Spammy solution. I think there were better solutions suggested as NIPs in nostr NIP repo.
Yes, this is spam by design. 😂
And yes, there are better solutions already proposed. Onion routing was one. Chain-of-ephemeral-pubkeys was another. I could imagine something based on NIP-26 delegation, etc.