Your experiment would not prove that they are deliberately slowing older phones via updates. That's big part of the claim. Your experiment would only show that as you update, your old phones will get slower and slower.

I wouldn't be surprised older phones get slower with updates, in fact, that feels like the most likely scenario for me, based on my experience as a software developer.

But IMO, Hanlon's (maybe Occam's, too?) Razor applies. Most likely, the teams just need to ship features, make fixes, and they mostly test on higher end, recent devices. Sure, at some point, someone tests on a lower end device that everything still works, but they probably either do not notice the issues, or shrug it off, or rationalize it (it might make x worse, but users get y in exchange, so it's fine).