I think excalibur meant "when the software version is two versions back", not the hardware. I suspect the family is still getting security updates because you

I think excalibur meant "when the software version is two versions back", not the hardware. I suspect the family is still getting security updates because you are running iOS 11 on the 5S. I bought a 5S too back in 2013, it came with iOS 7. If you were still running iOS 7 (or 8, or 9, or probably even 10 now) on the device, you would not be getting updates any more.