Along with iOS 11.4 Developer Beta 5, Apple released watchOS 4.3.1 beta 5 for Apple Watch on Monday. The update doesn’t seem to have new features and looks like primarily a collection of bug fixes.
Beta watchOS updates are available only to paid participants in Apple’s developer program. You can, however get public release versions of iOS, macOS, and tvOS betas. Those typically follow within a couple days of the developer beta releases.
Apple also released macOS High Sierra 10.13.5 developer beta 5 today.
If you’re registered in Apple’s developer program and interested in testing watchOS 4.3.1 beta you can check it out at the Apple developer website.