iPhones still get many years of security updates after they stop supporting the last iOS version.

> If effectively means it is absolutely unsafe to use anymore. Security bugs are found in browsers all the time.

Aren't the base operating system and the browsers (installed through an app store of some kind) separate? Even if the phone's operating system is not receiving any more updates, there will still be updates to the browser for a while. For instance, a quick search tells me Firefox is still being updated for Android 4.1, an operating system which is long out of support.

On iOS the browser (Safari) is tied to the operating system version. Alternative browsers still use the same rendering engine as Safari, which means they can't patch bugs in the engine, they need to wait for Apple to do it in a new operating system version.

I am talking about Apple devices here (edited for clarity).