Upthread at the moment there is a Samsung owner who does have shutdowns from battery problems.

Part of it has to do with how the battery has been treated. Wild swings in charge daily cause damage. Hot temperatures cause damage. I'd guess most iPhone owners have little to no damage. But for those who do, slowing the phone and giving a warning is a good alternative to shutting down.

I had plenty of reboot, freezes and boot-cycles on my not even 1 and half year old nexus 5. Maybe you just use your phone to do an odd call or two if you never experienced anything strange?

Do you live somewhere cold? My old devices are perfectly fine indoors but if I step outside in the winter they shut off in a few minutes

Li-Ion batteries lose the ability to provide energy at low temperatures, pretty fast. If the battery is aged, the effect becomes much more pronounced.

I've had my HP laptop running Windows (one of the best models a few years ago) shutdown randomly because of an aged battery. Replacing the battery made it usable again. I don't believe this is an Apple-only issue or even a smartphone-only issue. If it hasn't happened to someone, it may be due to many other factors at play.

The parent post actually mentions this happening on the 5S, a phone which did not get a software update choosing stability over performance. The 6 can slow down and run with a 20% battery while the 5S runs fast and must shut down due to the voltage drop.