If iOS 11.1 can be considered "the first service pack", then even that sounds iffy. Or at least, what I'm reading suggests that even with that update, my battery life would still be crap should I decide to upgrade.

Though this gets to another problem with Apple's maintenance of iOS: If you don't upgrade to iOS 11, you don't get their fix for krack attacks. Microsoft, on the other hand, did have the common decency to continue issuing security patches for older major releases.

This. When Apple devices reach two generations old, they need to be forcibly removed from your environment, or they will quickly become gaping security holes. For a company that prides itself on its image of protecting users' privacy, it sure is behind the curve on software maintenance.

My family has 4 year old iPhone 5S in service that still receive all security updates.

Our Android devices on the other hand... not so much. Six months of updates seems to be about the max!

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.