Linux

It's likely CentOS 7.9, which was released in Nov. 2020 and shipped with kernel version 3.10.0-1160. It's not completely ridiculous for a one year old POS systems to have a four year old OS. Design for those systems probably started a few years ago, when CentOS 7.9 was relatively recent. For an embedded system the bias would have been toward an established and mature OS, and CentOS 8.x was likely considered "too new" at the time they were speccing these systems. Remotely upgrading between major releases would not be advisable in an embedded system. The RHEL/CentOS in-place upgrade story is... not great. There was zero support for in-place upgrade until RHEL/CentOS 7, and it's still considered "at your own risk" (source).

It’s the kernel version

CentOS 7, released with Linux 3, only went out of support a month ago.

Expect to see it pop up everywhere the coming five to six years. The sort of people who still run CentOS generally aren't the sort of people who upgrade their software unless they absolutely have to. It's like those ATMs running that obscure version of Windows XP and 7 (although the Windows 7 Embedded release will still receives updates for a few more months)), you deploy an OS onto them, kill any accessible port you can find, and just let it do it's thing for a couple of years.

The only reason the DOS POS systems are being phased out is that the hardware they run on can no longer be replaced easily, and even then Dosbox is sometimes used to run POS software.

It's CentOS 7.x

It's not CentOS 3, it's CentOS with Linux kernel 3.10 (a 2014 kernel). This was supported in RHEL/CentOS through 2017.

Still very dated and a bad idea, of course. And even weirder that it's on a new machine. I've seen tons of stores using Win7 past it's EOL, but on older hardware.

What is the risk if it's all on a closed internal network? You can safely run Windows 98 as long as you're very careful about what goes in and out at your gateway.

Commerical networks tend to be a bit more robust than Joe Schmoe's basement router. It's a giant pain to keep up with each and every update on everything a store uses (not just self checkouts, things like CCTV systems, HVAC monitoring, electronic signage like smart screens, etc.) so usually it's all controlled at the network level.

I install CCTV, I guarantee you more than three quarters of the DVRs and PoE cameras I install never get updated and are "set and forget". I've pulled out 10-15 year old cameras still with original firmware in giant national chain stores when they do refreshes of their infrastructure.

Is CentOS still getting updates? I thought it’s killed by Redhat years ago.

I think I recognize that system by the monitor glare, where’d you take the picture?

a lot of stuff at my work uses a custom OS built in the 90s