iOS 13.1.3 was a minor update released in mid-October with bug fixes for issues with Phone, Mail, Health, iCloud, and more.
Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date. Apple has also stopped signing iOS 13.1.2, which was another minor bug fix update.
iOS 13.2 is the current public version of iOS, and it brought support for AirPods Pro, included new emojis, added Deep Fusion for the iPhone 11 and 11 Pro, and more.
Developers and public beta testers can also download iOS 13.3, an upcoming update that introduces Communication Limits for Screen Time.