The scuttleverse [1] comes to mind when you mention this. It allows for creating a decentralized mesh network with communication between each node end to end encrypted. It's able to operate off the grid and allows communication through Lan, Bluetooth etc. Happens to also work offline as the content is stored on your device and any posts you make sync up with others later. I'm not sure about the scalability of it though.
It's also available on mobile [2].
[1] https://scuttlebutt.nz
[2] https://www.manyver.se
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