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....
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.
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.