The more I read about some of the operating systems eventually died out (mostly), the more disappointed I am that the only realistic options for workstation/server OS’s are Windows and a handful of Unix-like systems.

I would be very curious to see a list of features which were available in defunct OSes that are not available or only available via kludge these days. There are some great ideas still out there in the wild.

I've heard wonders of Lisp OSes, but haven't tried one.

There’s also Mezzano which is a relatively new, from scratch Common Lisp implementation on bare metal: https://github.com/froggey/Mezzano

Finally, if you know where to look, it’s possible to setup a Symbolics VM on Linux, which is somewhat interesting to poke around in (all the manuals available out of the box are pretty interesting to read through.)