I have a 9 year old phone (Galaxy Note 4, yay for removable batteries!) as a daily driver, now on LineageOS and while it is true that removing the bloat will help a lot, it is very clear that the slowdown is real.

Among the most noticeable worst offenders is Google Maps, but many apps get slower update after update while not offering much more than cosmetic changes. Websites too, many apps are just websites bundled with a browser anyways. Apps that didn't receive updates for many years are as snappy as they ever were.