After acquiring Dark Sky in March 2020, as of January 1, 2023 the Dark Sky app is no longer available. Apple will continue to support the Dark Sky API until March 21, at which point it will also be switched off.
Apple has also published a new support guide for those looking at how Apple Weather can replace Dark Sky:
Dark Sky’s features have been integrated into Apple Weather. Apple Weather offers hyperlocal forecasts for your current location, including next-hour precipitation, hourly forecasts for the next 10 days, high-resolution radar, and notifications.
Apple used Dark Sky technology to roll out WeatherKit in 2022, which is now in use in the Apple Weather app across iOS, macOS and more. A REST API is also available, and Apple is encouraging developers to migrate immediately to avoid problems once the Dark Sky app is switched off in March.