It's no surprise that older Apple devices are at a disadvantage and will need to be kept updated as much as possible. It looks like there's another major reason to update your iPhone, especially those 2012 and older; your GPS system is at risk.
Apple has released a blog post urging users to update to avoid issues with location, date and time synchronisation due to the GPS epoch rollover. To put it simply, GPS systems depends on your location to get highly accurate time to properly function, with time in this context represented in weeks. The old systems use 10bit data to observe time, which means it can only reach a number of 1023. Once the number hits 1024, which is the maximum, the system restarts.
This will likely mean the device can't tell your location or even time accurately, but most importantly it could lead to the software being corrupted. As such, the devices that should be updated are those from 2012 and older, including the iPhone 5 and iPad 4th Generation (iOS 10.3.4), as well as the iPhone 4s, iPad mini 1st-Gen, iPad 2, and the iPad 3rd-Gen (iOS 9.3.6).
If you have one of these devices, you have a time limit of 12AM UTC (8AM in Malaysia) on November 3 to update your device. If you're affected, you will need to update your device in iTunes of Finder via a tethered connection. Failure to do so will mean you won't get OTA updates to path your system.