Personal experience, I have an iPad Mini (original) that has gotten slower and slower with each OS update. I should have just ignored all the updates and left it on iOS 6 or 7, but it's impossible to roll back now.
Most blatant example of planned obsolescence I've ever encountered. Apple even promotes it under the guise of "long-term support" for their older devices. I'm sure the idea was to get me to toss the thing and buy a new one, but instead it's made me not want to buy another Apple product again. At this point it is used for exactly one thing, Facetime, since there is no better alternative on Android.