Actually Apple have only admitted doing this for the last year or so: they say it started with iOS 10.2.1. I assume they are telling the truth there, because it's an open invitation for someone to go out and repeat the tests with an older version, so would be a stupid thing to lie about.
But complaints about iPhones slowing down when the OS is upgraded are nothing new. So I would assume that's because the newer version is more demanding and so the phone runs slower (whether they make less effort to optimise for older hardware would be a matter of pure speculation, and impossible to prove either way, so I suggest we don't go there). Although it's not a universal rule, we can all think of many cases of people complaining that their android device runs slower after an update, or people who refuse to upgrade their OS because they've heard that's what happens, so I'm not sure we can draw a clear distinction between platforms there.
But complaints about iPhones slowing down when the OS is upgraded are nothing new. So I would assume that's because the newer version is more demanding and so the phone runs slower (whether they make less effort to optimise for older hardware would be a matter of pure speculation, and impossible to prove either way, so I suggest we don't go there). Although it's not a universal rule, we can all think of many cases of people complaining that their android device runs slower after an update, or people who refuse to upgrade their OS because they've heard that's what happens, so I'm not sure we can draw a clear distinction between platforms there.
