In 1999 my team at Williams Electronics chose XINU as the O/S for the Pinball 2000 project:

https://www.youtube.com/watch?v=8rHbsCyde1g

At the time, embedded X86 boards were not really a thing yet so we needed something simple that could run with a COTS PC motherboard.

The major factor was that the licensing for other operating systems was too costly or too restrictive. Anything GPL-based was ruled out by our lawyers. Turning Linux into an RTOS would have forced us to release source code and that wasn't a fight with Legal that anyone had time for.

XINU turned out to be just what we needed. The process limit wasn't a huge problem since we were used to running threads on a single process. Adding networking later turned out to be very easy.

Did you look at MINIX?

I don't recall. The lead O/S engineer was from Purdue so perhaps he went to XINU right away out of familiarity.

They have a whole list on that page. Lexmark printers in the 90s, for instance, used Xinu.

Interesting, I worked at Lexmark for a few years and didn't know that. One of my professors at the University of Kentucky worked on Xinu, I wonder if he had any influence on it being used at Lexmark (the university and company are both based in Lexington, KY)

Printers sold by Lexmark until 2009 used Xinu onboard. Now they're some lightweight Linux.

I expect that it's role in the OS ecosystem is mostly taken up by projects like FreeRTOS.