Apple gives some older iPhones OS updates, going back to iPhone 5s
https://bit.ly/3kESWfn
给旧机器的安全更新，有 iOS 12 跟 15，为什么只有这两个内文也有讲了
"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 年前的机器 (iPhone 5s) 也算够有诚意了