Apple iPhone 5s might get iOS 12 according to WebKit test26
But thanks to the open source nature of WebKit, the browser engine used by Safari, we were able to find a page that mentions a test run in January that included an Apple iPhone 5s running iOS 12. This is not a guarantee that those with the iPhone 5s will get one more year of running the most up-to-date version of Apple's mobile operating system. But it should give some hope to those still sporting the phone.
We should know for sure when iOS 12 is unveiled during WWDC 2018. The annual developers conference begins June 4th and runs through June 8th, and will take place at the McEnery Convention Center in San Jose, California. If you own an iPhone 5s, keep your fingers crossed.
source: Macgeneration (translated), 9to5Mac