Do you have a source for this by chance?
For instance, a restarting computer at the local Kroger where I shop said IBM SurePOS. Looking it up, it turns out some group tried to make a concurrent version of DOS that IBM modified into a specialized OS for POS's. Recently, they upgraded to touchscreens running Linux on Fujitsu boxes. The GUI is ugly, slow, and has the tell-tell coffee mug in corner of dialogs. Appears Kroger, always slow as can be, got on Java desktop bandwagon just as everyone else chasing fads were leaving it for stuff like web apps or Electron.
The self-checkouts have a Windows loading screen. Probably was WinXP Embedded given they're old. The handhelds use mainframe-era, terminal apps. Their computers at customer service are thin clients from HP at this location. I think one said "Iceweasel" one time. The checkout predictor thing with the three balls on monitor showed something Linux-related one time. If you ask employees, you'll get SCO OpenServer, AS/400, mainframe, and a pile of random apps with web interfaces. I don't know what they run. One employee told me email and web browsers load in what must be VM's that take 5-10 min to load on the thin clients. It must be unbearable.
So, Kroger certainly uses plenty of Linux with mission-critical services being on it. Their choices for inventory and desktop GUI's also hint they're pretty backwards in their practices. I'm not sure about the others since I mostly shop at Kroger.
I think you really do not know much about the history of PC operating systems.
IBM licensed Digital Research's real-time OS, FlexOS, for its point-of-sale machines.
https://en.wikipedia.org/wiki/FlexOS
FlexOS is related to DR's Concurrent DOS:
https://en.wikipedia.org/wiki/Multiuser_DOS
... which was the descendant of Concurrent CP/M, which was the offspring of CP/M-86, one of the 3 original OSes at launch for the IBM PC in 1981 (along with PC DOS and the UCSD p-System).
CP/M was the original industry-standard OS for 8080 micros. It is what MS-DOS is a knock-off version of -- MS-DOS was licensed from SCP, whose QDOS was a copy of CP/M.
"Some group tried," indeed. This is the original OS company, successfully trading and dominating the industry when Bill Gates and Paul Allen founded Micro-Soft.
Well, that's a big assumption and rude at the same time. I post a lot here about the history of operating systems. I'm well aware of DOS and CP/M. I know little to nothing about Concurrent DOS or FlexOS because of no need. So, let's look at your link:
"IBM licensed Digital Research's real-time OS, FlexOS, for its point-of-sale machines."
...the article you link says "IBM originally chose DR Concurrent DOS 286 as the basis of their IBM 4680 computer for IBM Plant System products and Point-of-Sale terminals..." The original source I read those terminals said they were based on Concurrent DOS. So does my comment and this link. Clearly, I did some research. Kroger's POS's also said IBM 4680.
Let's see where FlexOS comes in: "IBM announced the adoption of FlexOS... as the basis of their IBM 4690 OS..." Ok, FlexOS was used for a different one I didn't look up because Kroger didn't use it.
So, you're saying I'm ignorant about OS's because I didn't study a proprietary OS (FlexOS) I've never seen, the local store didn't use, and that was likely inferior to most RTOS's I know of? I'm not sure what your aim was.