This is a common claim but I rarely see it supported. My iPhone 6S from 2015 is fully supported by iOS 15[1]. I know plenty of people whose cars had day 1 drawbacks in the on-board software which were never fixed (or required paid upgrades in the $500+ range) — and that includes things like “the entertainment system will crash every time you connect your phone because you have more than a small number of songs / contacts” — and most people I know who own cars upgrade them more frequently than every 6 years.

1. https://support.apple.com/guide/iphone/supported-models-iphe...