Even if Apple sets an "expiration date" for all marketed devices, touching both the software side and the hardware side, the company is recognized for the constant delivery of firmware updates as long as the hardware can keep up with the innovations.
Officially, iOS 11 is the latest major update available for iPhone 5S phones as well. Unofficially, Apple seems to be working on updating iOS12, the clues found in the WebKit source code (using the Safari browser) showing compatibility between iOS 12 and iPhone 5S phones
Apple may have brought iOS 12 for the iPhone 5S to the testing phase, and it remains to be seen if the performance of the first 64-bit CPU iPhone can be optimized for the new software
Including the iPhone 5S on the list of devices that get upgrade to iOS 12 brings back to recent speculation that Apple has focused its efforts more on optimizing performance and bug fixes, actually correcting the issues of the current iOS 11