When Dark Sky was acquired by Apple last year, it promised to keep the API functioning at least until the end of 2021. As of now, developers dependent on it have about a year and a half to find another provider.
There is a WeatherKit private framework lurking in iOS 15 that does not exist in iOS 14. It currently only contains strings of different weather conditions, but perhaps it will be more substantial and not private in the future.
For what it’s worth, there is a same-named private framework in MacOS Catalina and Big Sur, but its contents are very different. It contains images of different weather conditions, and lengthier sentences like “The high will be (placeholder). (Placeholder) tonight with a low of (placeholder).” instead of the simple condition text (“sunny”, “cloudy”) in the iOS 15 framework. Therefore, I do not believe it is a mistake in copying files from a shared code base.