> Slirp is a well-known project that was originally and mainly used for networking in QEMU (aka Quick Emulator).

Good grief, that's off by almost 15 years. Slirp was originally a SLIP endpoint you could run on a remote dial-in shell account to connect your local system to the internet. As it happens that's basically the same problem needed to get (something like) IP to work out of a userspace context in general, so it got appropriated by the container community and used pervasively. But it's roots go much deeper.

Also... "Quick Emulator"?!