Apple released iOS 13.3 back on December 10, and as is traditional, Apple has now stopped signing iOS 13.2.3, making downgrading to that version of iOS no longer possible.
Apple is known to stop signing older versions of operating systems after it releases a public update, as part of an effort to encourage users to keep their devices’ operating systems up to date.
iOS 13.3 is the current public version of iOS, and includes Communication Limits for Screen Time, which allows parents to control who their children can call, FaceTime, or Message. It also includes contact list for children lets parents manage the contacts that appear on their children’s devices.
The update also brought support for NFC, USB, and Lightning FIDO2-compliant security keys in Safari.
iOS 13.3.1 is currently in beta, and is being tested by developers and public beta testers.