Apple gives some older iPhones OS updates, going back to iPhone 5s
https://bit.ly/3kESWfn
給舊機器的安全更新，有 iOS 12 跟 15，為什麼只有這兩個內文也有講了
"Users may note the skipped iOS versions between iOS 12 and iOS 15.
Those are due to where devices were cut off from updating.
Every device that could run iOS 13 could run iOS 15,
so Apple doesn't update every version."
支援到快 10 年前的機器 (iPhone 5s) 也算夠有誠意了