Have you heard of [syncthing](https://syncthing.net/) ? It syncs a folder between devices, like Dropbox, without the need for someone else's computer. No cloud, it's just your pc talking to your phone, directly and automatically, through your home WiFi.
https://yggdrasil-network.github.io/
sorry for not providing a link. It’s not totally related to legacy ip, but might be interesting in the context of the whole topic.
tldr: it’s an encrypted mesh network on top of the internet (and every member gets an yggrasil ipv6 address)
Yes, that seems all like neat technology, but what is the use-case for that?