Apple would follow it up immediately with a 10.3.1 release. Especially without a beta testing period, this mysterious update naturally raised a few questions. Although Apple would later release a note on what the update fixes, a security issue related to Wi-Fi, it might have left out the equally important part about fixing the update would have broken the iPhone 5 and iPhone 5c.
iPhone 5s and its A7 CPU, Apple has been using 64-bit processors in its mobile devices. In fact, Apple is credited to have given the needed impetus to get that new phase rolling in the industry.
The problem is that iOS 10.3 still supports the iPhone 5 and iPhone 5c, Apple’s last 32-bit iPhones. The update, then, would have caused problems for owners of those older phones. Apple quickly yanked the update from its servers for those two models. With 10.3.1, that issue has presumably been addressed.
Officially, iOS 10.3.1 also fixes a rather severe security issue, one where an “attacker within range may be able to execute arbitrary code on the Wi-Fi chip.” It definitely sounds rather frightening, especially given that Apple rushed out the patch for it.