As software becomes more demanding, hardware needs to be upgraded in order for it to keep up. This is why as time passes, newer software updates are sometimes incompatible with older hardware. For example we’ve seen it happen with Rockstar’s GTA Online where the company stated that future updates would skip older consoles like the Xbox 360 and PS3.
The same can be said for smartphones too, but for the most part Apple has been pretty good with supporting older devices, even if it means that they do not get certain features. Unfortunately not everyone sees it that way because a class-action lawsuit has been filed against Apple, in which the complaint is that Apple had crippled the iPhone 4S with the iOS 9 update.
The iPhone 4S is actually four generations old so obviously the hardware is nowhere near as good as the iPhone 6s/6s Plus, which also means that iOS 9 might not be the optimal experience. This is the basis of the complaint in which they claim that iOS 9 had negatively impacted the iPhone 4S where it was deemed unfit for daily use.
The lawsuit also alleges that Apple knew about these limitations, but went ahead by pushing it out anyway and marketed the update as bringing about faster performance, better battery life, and etc. The plaintiffs are seeking over $5 million in damages with an option to treble.