First watchOS 4.3.1 beta includes a warning about support for older apps3
It was last November when Apple told developers that sometime in 2018, it would no longer accept watchOS 1 app submissions. Since the first of this month, Apple has required developers to use the watchOS 2 SDK to update older apps. New apps must be built using watchOS 4 SDK or later.
While the warning doesn't mention the specific date when support for the watchOS 1 legacy apps will end, there is some speculation that this will take place with the launch of watchOS 5. The new build of the Apple Watch operating system is expected to be unveiled at WWDC 2018. Apple's annual developer convention will run from June 4th through June 8th at the McEnery Convention Center in San Jose, California.