Apple has moved on to its sixth round of betas for this current generation, issuing a new build of just iOS 12.3 to developers to try out on their devices, with the single beta build possibly a sign a full public release is imminent.
Developers taking part in the program can download the latest test build via the Apple Developer Center, or receive it as an over-the-air update on enrolled devices. Participants in the public Apple Beta Software Program can usually expect a similar beta update to be issued shortly after the developer version.
The sixth beta build of iOS 12.3 is number 16F5156a, replacing the fifth build, 16F5155a.
Apple issued the fifth builds of this round of betas on May 7. It is unclear when the sixth betas of tvOS 12.3, watchOS 5.2.1, and macOS 10.14.5 will be released, if at all.
The main changes in the betas revolve around the updated Apple TV experience, with tvOS 12.3 and the TV app in iOS 12.3 being updated in preparation for the formal launch of Apple TV Channels and the Apple TV+ services, which should arrive later this year. Outside of TV, another notable change relates to the Wallet app in iOS, which is gaining additional features so it will be ready for the US launch of Apple Card this summer.
AppleInsider, and Apple itself, strongly suggest users avoid installing betas on to "mission-critical" or primary devices, due to the potential for data loss or other issues. It is instead recommended to install betas onto secondary or non-essential devices, and to ensure there are sufficient backups of important data beforehand.
Update: Apple has issued a public beta version of the latest iOS 12.3 evaluation release.