QNX springs to mind in that case, but I'm not sure if it's really viable as a day-to-day OS. There's a distro of it out there somewhere, I guess.
(Usually it's used in embedded systems)
(Usually it's used in embedded systems)
For what it's worth, QNX <7 is similar to Windows XP - not the latest version, but very much in active use, and definitely not abandonware.
My interest in QNX is solely academic/hobbyist; I actually keep QNX 4.25 and 6.5.0 open in QEMU in the background and VNC to them occasionally for fun. (4.25 uses 1.3% and 6.5.0 uses ~5% CPU if I don't pause qemu.)
Finding older versions of QNX is a small challenge; getting them working is also Fun™. For reference, my email is in my profile.
For reference, older versions of QNX may not work well on newer hardware, unfortunately.