If you use any apps on your Apple Watch that haven't been updated for newer versions of watchOS, you might be leaving them behind in the near future. In watchOS 4.3.1, the latest beta version, Apple Watch users are being notified that apps built for watchOS 1 will not work with future versions of the operating system.
The following message (as noticed by Appleinsider) pops up when you open one of these apps:
This app will not work with future versions of watchOS. The developer of this app needs to update it to improve its compatibility.
Tapping "OK" under this message will allow the app to run as it normally does.
While we don't know which "future version" of watchOS will drop compatibility for older watch apps, it's likely that this message refers to watchOS 5. Assuming that Apple follows the patterns of previous years, watchOS 5 will be unveiled at WWDC 2018, with a release for Apple Watch this fall.