SimpleX said at https://simplex.chat/blog/20240314-simplex-chat-v5-6-quantum-resistance-signal-double-ratchet-algorithm.html#end-to-end-encryption-security-attacks-and-defense: “SimpleX Chat does not use signature in any part of client-client protocol, but the signature is currently used when authorizing sender’s messages to the relays. v5.7 will improve deniability by enabling a different authorization scheme that will provide full-stack repudiation in all protocol layers”, however there is not even the word “deniability” or “repudiation” in the “What’s new in v5.7” blog post https://simplex.chat/blog/20240426-simplex-legally-binding-transparency-v5-7-better-user-experience.html which is according to the first quote the version where the improved deniability came in place and I would therefore like to ask if this is now really implemented?
You are thinking of cryptographic deniability but there is also plausible deniability to consider. You may be interested in this post about deniability.