Up til 20 years ago there were a surprising number of ATMs still running OS/2; NCR and Diebold supported old machines for a long time. Especially small market/small regional banks wanted to get the absolute most out of their capex investment. Over the years, I've worked with a couple of those dead-enders on different GRC projects, mostly because I'd actually seen OS/2 before. AFAIK, those vendors stopped supporting OS/2 in the 2000s; I'd be very, very surprised if there were any left now.

I you're interested in how a very "not Unix" operating system is architected, I really recommend Deitels' "Design of OS/2". Very interesting.