Harvard study finds correlation between iPhone launch times and user complaints of slowing software performance on older models.
http://www.notebookcheck.net/Evidence-suggests-Apple-may-be-purposefully-slowing-down-older-versions-of-iOS.154583.0.html
That's a very stupid analysis.
Appe releases the major iOS versions with the new iPhone launch. So, during that time, a lot of users of older iphones get the new iOS which may slows down their device a bit since it brings more eye-candy, more features,...
In additions, dveloppers will update their applications at the same time, taking advantage of new APIs, higher resolution, ... which makes the applications heavier and thus potentially slower.
Samsung does not release a new version of ther Android based OS with the release of a new Galaxy model, for instance the S6 did not get android 6 with the release of the note 5. So the comparison is quite frankly ridiculous.
Your comment is frankly ridiculous as well. Samsung does not release any new Android version as long as Google does not release a new one to base their TouchWiz UI on. I think Apple is large enough to afford support for older devices, so the OS can work the best way possible on old hardware, don't you think? Look at Windows 10, for example - it works great on my 6-year old laptop.