Your iPhone 5 will stop working soon if you don't install this update
According to the company, iPhone 5 units that aren’t updated to iOS 10.3.4 by the end of next week will be unable to connect to the internet. Basic services such as the App Store, iCloud, email, and browsing the web via Safari or Chrome will stop working.
Apple originally stopped development of iOS 10 two years ago following the release of iOS 11 but was ultimately forced to correct an important issue this summer which helped maintain compatibility of older iPhones and iPads with GPS networks as they go through a time rollover transition.
From what’s currently understood, fourth-generation iPads are also affected by the change although the only functionality that’ll be affected is GPS. Either way, it’s probably a good idea to install the update.