Apple would no longer want to provide the iPhone 6S and SE (2016) with upgrades. That claims an Israeli site. They are currently the oldest phones that have gotten iOS 14. The phones came out in 2015 and 2016.
The oldest phone with iOS 15 would therefore be the iPhone 7 from 2016, reports The Verifier. The site previously correctly reported early on which phones would support iOS 14 and iOS 13. The Israeli site also has a shaky track record of Apple rumors.
IOS 14, released this fall, supports all devices that also got iOS 13, while iOS 13 ended support for the iPhone 6. The iPhone 6s came out in 2015 with iOS 9, the SE followed in March 2016 with iOS 9. Both devices would be stuck on five version upgrades. The iPhone 6 got four version upgrades before support for new iOS versions ended, the iPhone 5s also got five, and the iPhone 5c got three version upgrades.
Apple is known for its relatively long support for smartphones; with Android phones, the number of version upgrades is usually between one and three; for some cheap smartphones, the manufacturer does not release any version upgrade, while models from Google and more expensive phones from Samsung, among others, come with three version upgrades before the support stops.