Here the email I’ve just received:
Hi there,
We have some important news to share: Dark Sky has joined Apple. You can read more here: https://blog.darksky.net.
The self-service API for existing customers is not changing today, but we will no longer accept new signups. The API will continue to function through the end of 2021.
Use of the API service is subject to the updated Dark Sky Terms and Conditions, including the Apple Privacy Policy ("Terms"). By continuing to use the API service, you are agreeing to be bound by these Terms.
If you have any questions, please contact us at developer@darksky.net.
- The Dark Sky Team
It seems game over end of 2021. 
Any other good weather API online?
Andrea