Wut? Can you explain for dumb people?
https://en.wikipedia.org/wiki/AppleTalk was very thorough on the protocol's history, but I was really puzzled by the purpose and why somebody would re-implement a discontinued protocol.
UML is full Linux running as a userspace process. Tools like guestfish use it to modify disk images. UML can only run on Linux though.
LKL lets you use the linux kernel as a library. However, only single threaded. But this allows you to use linux's tcp stack from userspace, or its filesystems, etc. LKL also works on non Linux platforms.
These are definitely a different shape than what you're looking for, but they could help you keep the real linux implementations alive from userspace far into the future.
I have a long discussion thread here which chronicles the development of this stack, and the two little hardware projects that go with it: https://68kmla.org/bb/threads/tashtalk-usb-tailtalk-and-inkt....