You wouldn't really want to mess with the OS in cases like that, though. If you did, a lot of the OS-included drivers would probably not work due to removal or bit-rot, or 3rd-party drivers might not work with a much newer OS. The only reason you might want to upgrade is if the machine is Internet-connected, in which case you're probably just going to need a whole new box (if you need the old software, you'd probably resort to a VM), otherwise the new OS will run too slow (especially with things like new crypto/security). Or you can keep the old box but put it behind nginx or whatever.
Point of sale or industrial control computers are kind of just an offshoot of "embedded"; not something you need (or want) to run a browser or a modern GUI[0] on. You stick it in a cabinet or a kiosk and hopefully you don't have to think about it again.
[0] sadly also a browser