Briar is a messenger app that worked on local networks, over Bluetooth, and over Tor if traveling the Internet. Fully encrypted and the purpose was decentralized, serverless messaging.
I liked the concept, and tested it out a little on my Android devices. But it looked straight out of 2009, and it had the issues described in the post. Still. Thanks for the work. I hope it can get revived or inspire others some day.
P.S. feature request! If Alice, Bob and Charlie are all contacts with each other, and Alice writes an offline message to Charlie, Alice should be able to opportunisticly hand the encrypted message to Bob on their shared network, and Bob can deliver it to Charlie.