It seems to be pretty popular to create “fantasy consoles”, ie something like an emulator but for made-up 8-bit game consoles. Pico-8 is probably the most known one, but here’s a long list: https://paladin-t.github.io/fantasy/ But are there any *fantasy operating systems”? Ie not so game-oriented, more exploring alternate history ideas for say WIMP-style OSes, preferably serious enough that you could make “real” software for it.

Many people run old, simple operating systems in emulators or sometimes directly on hardware. For example, my friends at Hundred Rabbits are currently using Plan9 on a Raspberry Pi as their primary development system: https://twitter.com/hundredrabbits/status/1290361357888417793?s=20

There are definitely some who still use Plan9, and its editor Sam.