Apple has now made the iOS 12.4, tvOS 12.4, and macOS Mojave 10.14.6 betas available for the public.
According to MacRumors, all three beta builds focus on bringing under-the-hood improvements rather than showing off new features.
Apple users who want an early taste of new software should proceed to Apple Beta page and follow the instructions on how to install the public beta builds.
Both the iOS 12.4 and macOS 10.14.6 betas could be the last updates for the current software generation. Apple will likely move on to the darker iOS 13 and a more versatile macOS Catalina after kissing goodbye to iOS 12 and macOS Mojave.
As an important side note, iPhones older than the iPhone 6S or iPads launched before the iPad Air 2 won't get iOS 13.
Source: MacRumors, (2)