I feel like there was an inflection point a few years ago where smartphones were fast enough that even half a decade of OS updates couldn't cripple it. I distinctively remember all the iphone 3GS/4/4S out there that were unusable after 3-4 years of updates. Whereas iPhone 6s/SE (they use the same chipset) after 5 years of updates is very usable. The difference is day and night.

Depends on which one.

The iPhone 6 lost support in 2020 or 2021.

The original iPhone SE (the little one from 2016) is equivalent to a 6s (both use A9) and it’s widely expected that Apple will drop support this year.

However the newer/bigger iPhone SE that was released in 2020 has the same chip as the iPhone 11, the A13. It should still be good for many years.

Why did you stay on such an ancient OS?

You likely are using a version of the App Store which doesn't support pinning older OS releases on older app releases either -as soon as they released a build for iOS 7, the iOS 6 version would no longer be accessible.

You are also missing about seven years of security updates, including several browser-accessible remote exploits. Not using Safari would be a good idea.