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

It's CentOS 7.x

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?