Apple has quit signing code for iOS 13.6.1, meaning that users who updated to the latest iOS 13.7 operating system from downgrading.

iOS 13.7 was launched don Sept. 1. It includes the Apple-Google develops COVID-19 notification system. 

The exposure notification system uses Bluetooth signals from smartphones that have opted-in to determine how closely and for how long two phones were nearby, without collecting the location or identify of the users. The ability to opt-in to the system without downloading an app is intended to improve adoption of the system, which would improve its effectivity, as it requires a large number of people sharing Bluetooth signals to work.