Although older iPhones don't support the latest version of iOS nor do they have any of the new features, they should continue to work for a while. As long as your iPhone (older versions) can keep up with iOS software updates, you'll be ready to use your iPhone for a couple more years. Apple will never stop supporting its smartphones until it's an older iPhone model, as Apple offers software updates every year and every software update needs compatible hardware parts. Apple will not stop supporting any version of the iPhone until the iPhone (and its hardware) are competent enough to update the operating software.
You can continue to use the iPhone 4S and it will work efficiently, except for the fact that if you face any problems, you won't be able to get help from the real Apple stores or Apple support. If your iPhone doesn't appear in the list above, it won't be compatible with iOS 17 and you won't receive a notification to download the latest software. We're going to collect all the historical data on the compatibility of each iPhone model, along with release dates, discontinuation dates, and iOS compatibility. So, we can see that the demand for the iPhone XR has undoubtedly not diminished compared to older iPhone models.
Apple has promised to provide support for the iPhone 6 and its plus models until the release of the iOS 16 updates. Apple's iPhone 6 plus model can work effectively with iOS 14 updates, just as the iPhone 6s plus will work alongside iOS 15 updates. Apple stopped offering feature updates for the iPhone 6 and iPhone 7 series with the release of iOS 16 last year. According to Apple, they would provide a five- to six-year upgrade for any iPhone launched on the market, meaning they would provide service for an iPhone 11 for up to seven years.
In case your iPhone 12 pro is unable to run the latest iOS update (as some users are having problems), you can always contact Apple support. In addition, the iPhone 5S is not compatible with the widely used 4G LTE network, as it works predominantly on a 3G network.