The original version of the iPhone SE and the iPhone 6s are six years old now.
They run the current version of iOS and still work just fine.
My guess is many have had something go wrong to the point that they're no longer used.
Even the performance under the current OS, six years later, is still fine.
https://arstechnica.com/gadgets/2020/09/ios-14-on-the-iphone...