This work-in-progress feature is meant to replace the current DM system. The Olm library is being used in production by Matrix, although I believe app developer

This work-in-progress feature is meant to replace the current DM system. The Olm library is being used in production by Matrix, although I believe app developers could use the libsignal library in their apps all the same. Please mind that neither Olm nor libsignal are actually included in the PR or used server-side in any way. The API is just key exchange and passing encrypted blobs. Message franking is a technique used by Facebook in their own E2EE chats that allows them to trust user reports about E2EE messages, otherwise someone could claim they received something abusive and there would be no way to know it's true. Believe it or not, "just block the sender" is not a sufficient solution because while it works for the individual, when a spammer is allowed to run rampant and thousands of users receive spam messages from different spammers regularly it brings the quality of user experience for the whole platform down significantly.