When have they done that? Most throttling over time is due to 2 reasons:

1. batteries degrading over time store less energy, so there’s less energy for the internal components. Workarounds can involve software throttling to avoid shutdown issues (which can corrupt your data) but the ultimate solution is a battery replacement. This isn’t inherent to software updates, it’s a consequence of a physical component breaking down over time.

2. engineers can choose to spend time maintaining compatibility with each new software release, or focus on optimizing for new hardware, or a little of both. Over time it’s possible later OS’ are less optimized than newer devices but this is not exclusive to Apple, and as many here have already pointed out Apple is way ahead of the competition in legacy device support already.

Can you prove that Apple has ever intentionally “slowed down older phones” in any other situation?