iphone is very popular phone you always want to buy it when it will become obsolete
Apple products become vintage when they have not been manufactured or sold for 5-7 years. They become obsolete when they have not been manufactured or sold for 7+ years. If you go to support.apple.com/en-us/HT201624 you can find lots of information about Apple's Vintage and Obsolete products. That being said, some devices will begin to stop supporting iOS updates due to incompatibility issues. For example, if you go to support.apple.com/en-us/HT210327 you can view a list of devices that are compatible with iOS 13 and iPadOS. iPhone 6s and 6s Plus are still compatible with the current iOS.