Apple iPhone 5s doesn't receive the iOS 11.3 battery health features
Besides offering $29 battery replacements until the end of the year (reduced from $79) the company added certain tools in iOS 11.3 that allow iPhone users to monitor the health of the battery in their handsets. The update also allows owners of these models to block Apple from throttling the CPU on their phones, no matter the condition of the battery inside it.
Apple is offering these tools to those who own the Apple iPhone 6 and newer versions of the device. However, the company did say that it has employed more advanced hardware and software in its 2017 phones (iPhone 8, iPhone 8 Plus and iPhone X) "that provides a more accurate estimation of both power needs and the battery's power capability to maximize overall system performance. This allows a different performance management system that more precisely allows iOS to anticipate and avoid an unexpected shutdown."
More interesting is the fact that while the Apple iPhone 5s received iOS 11.3, the oldest iPhone model to do so, it does not come with the battery health features. That is because Apple says it does not officially throttle the CPU in that device. The company says that the issues that forced them to slow down the processor inside older models are first found in the Apple iPhone 6. So even though the latter model is a year younger than the iPhone 5s, the older model is built differently and can squeeze enough power out of an elderly battery to run complicated tasks without shutting down the phone.