The QNX 4 demo was astonishing, but after QNX 4 they skipped 5 and 6 was a lot bigger already. One important developer also passed away back in those days (~2001).
There was also BeOS and Amithlon [1], AmigaOS running on x86-32 (IIRC booted up with some patched Linux kernel). Archive.org supposedly preserved this. [2]
20 years ago, one could run a Linux firewall on a floppy. One good thing about that, is that you could have a floppy physically put to read-only mode (SD cards also have this feature).