iOS 12 will continue to support iPhone 5s
Apple’s next-generation mobile operating system, iOS 12, has appeared in the WebKit development test report. Based on the information found so far, iOS 12 will continue to support iPhone 5s, which is good news for older devices. However, after the iPhone 5s is upgraded, the fluency of the system is certainly not guaranteed.
The bug report section of the open source Webkit project page explicitly mentions iPhone 5s devices running iOS 12. According to the Apple Support page, iOS 11 is compatible with iPhone 5s, iPhone SE, and newer models, but does not support iPhone 5c and iPhone 5.
Of course, it is also possible that iOS 12 support iPhone 5s is only for development and testing, and may be removed after the official release. Currently, there is no guarantee of which devices iOS 12 will be compatible with. Since WebKit is open source, it is difficult to keep future plans confidential in the WebKit community. Apple will release next-generation systems such as iOS 12, macOS 10.14 in June of this year.