Apple today announced that as of April 1, 2018, developers will no longer be able to update apps created for pro watchOS 1.
At that time, watchOS did not even have a complete SDK for app development. They were based on framework WatchKit and basically functioned as an interface for apps that ran completely on the iPhone paired with the clock.
Nowadays, apps run completely independently on the Apple Watch. And with watchOS 4, Apple was even more so because we now have GPS + Cellular watch versions that really can work 100% without an iPhone nearby.
From that date, therefore, Apple will only accept native apps and games on the watchOS App Store for watch. Which means, of course, that many apps that are out there today and have never been updated to the SDK should disappear from the map.
Taking advantage of the opportunity, Apple also announced that the watchOS app package can now be up to 75MB. Before, the limit was 50MB.