Today, Apple finally announced that it's going to stop accepting app submissions for apps that target watchOS 1. The rule has already been in effect for new apps, but beginning on April 1, 2018, all app updates will need to use the watchOS 2 SDK or later.
In the watchOS 1 days, all of the processing for a Watch app happened on the phone. It wasn't until watchOS 2 that the company actually delivered standalone applications, so it's completely reasonable that Apple wants to leave that time behind.
New apps, however, need to use the watchOS 4 SDK or later, which would target the latest version of the OS. Apple also announced that it's increasing the maximum size of Watch apps from 50MB to 75MB, and with the new Watch Series 3, developers can use new background modes for "navigation and audio recording, built-in altimeter capabilities, direct connections to accessories with Core Bluetooth, and more."
The current version of watchOS is 4.1, with the most recent 4.2 beta being released on Monday. While there are now three generations of the Apple Watch available, they can all be upgraded to the latest version of the OS.