@meko Have you heard of #NNCP? That's pretty much the best example of such a program that comes to mind.
https://www.complete.org/nncp/
https://nncp.mirrors.quux.org/
It doesn't require any connectivity at all in order to work.
#Briar is a bit different but also provides an off-grid messenger. https://briarproject.org/how-it-works/
Even #email tolerates disruption fairly well (https://www.complete.org/asynchronous-communication/).
@meko Some things like #LoRaWAN / #Lora that would be usable as a transport.
Unfortunately most #HAM stuff is subject to regulations that are stupid about #privacy & #encryption, so despite there being adequate #PacketRadio protocols available for #FileTransfer and/or #Filesharing, you can't actually use them for anything.