For other HNers, Apple intentionally slows down their older versions via updates. They've been fined for this in a variety of countries and courts in the last few years.

From Oct 2018: https://www.cnet.com/news/apple-and-samsung-fined-for-slowin...

It's straightforward to find other sources and examples.

Generally, you are correct, try to avoid updates after the first few months of purchase.