No, Apple is not slowing their phones intentionally. It's a classic category-mistake.
Apply Occam's Razor: The implication of the intentional slowdown hypothesis would require the silencing of thousands of engineers over the years and risking a near trillion-dollar market by a public company. Versus the much simpler explanation that Apple isn't overly concerned about new software not running optimally on older phones.
The latter is an entirely plausible dynamic: Apple (like other big tech companies) move fast and constantly evolve new abstractions (new SDKs, new features, etc.). And if a new feature runs fast on new phones but older phones suffer some single-digit percent performance penalty, that might be seen as acceptable. No one has intentionally made old phones slower, but the effect compounds over the years. And especially Apple is clearly running a strategy of constantly evolving and moving its ecosystem forward (unlike Microsoft, which insisted on 100% backward compatibility for decades). There are very nuanced tradeoffs in those opposing strategies so I'm not claiming one over the other, but I will die on the hill that Apple's success is partly shaped by its commitment to move everyone forward, because it's how they've kept their ecosystem of apps being reasonably well maintained. I think this same strategy also manifests in their hardware, e.g. they dropped disk-drives, USB-A, etc. before others.
