Hardware needs to be upgraded in order in order for it to keep up, as software becomes more demanding.
That is why as time passes, newer software updates are from time to time incompatible with older hardware.
For example we have seen it happen with Rockstar’s GTA Online where the company said that older consoles would skip like the Xbox 360 and PS3.
Exactly the same may be said for smartphones also, but for the most part Apple is pretty great with supporting devices that were older, even when it means that they tend not to get certain attributes.
The iPhone 4S is truly four generations old so clearly the hardware is nowhere near as great as the iPhone 6s/6s Plus, which likewise means that iOS 9 might not be the best experience. Here is the basis of the criticism in which they assert that the iPhone 4S where it had been deemed unfit for daily use had been negatively affected by iOS 9.
The lawsuit also alleges that Apple knew about these restrictions, but went ahead by pushing it out anyway and promoted the update as bringing about better battery life, faster performance, and etc. The plaintiffs are seeking over $5 million in damages with an option to treble.