Yes, mid weight embedded (bigger than RTOS, smaller than big embedded like commercial storage appliances) is probably its remaining commercial niche and it does

Yes, mid weight embedded (bigger than RTOS, smaller than big embedded like commercial storage appliances) is probably its remaining commercial niche and it doesn't have the current marquee usages that FreeBSD has in various products and infrastructures but I think one of the longest running is at IIJ. The 10.0 release is a massive improvement on modern hardware. It's worth a look, for learning purposes if nothing more, as it is pretty clean and accommodating of research like the linked article.