Since you seem to focus on decentralized protocols, I should add: In practice, while we all like federated and p2p apps for the freedoms & this warm fuzzy feeling they provide us with, by default they tend to have a much greater attack surface when it comes to metadata. This is because, compared to a centralized approach, metadata is openly available to far more parties. As a result, 3-letter agencies often won't even need a warrant to get their hands on the metadata: They can simply run traffic analysis and/or participate in the network themselves.