The 3.x branch is still under active maintenance both by the kernel devs until 2020 and by Red Hat for longer, 2024 as it's far down on the page. CentOS benefits from the maintence work put out by Red Hat, indeed it is now owned by them, so I would not yet call it obsolete.
For something like cash registers new features are not needed, and often are not even desirable. Embedded OS's are a different world, even when like this one they are adapted from general purpose ones.
However even outside of Embedded OS's some servers benefit more from a stable platform than any new features of the 4.x branches. That is the target audience of RHEL and CentOS.