After releasing iOS 12.1.3 with a few minor bugfixes, Apple has has moved on to iOS 12.2. The first beta was released to developers on January 24, 2019. Currently, there is no public beta release.
Registered developers can download the iOS beta profile by visiting developer.apple.com/download using the device on which they want to run the beta. Non-developers can enter the public beta program by visiting beta.apple.com on the device they wish to enroll in the beta.
