You can also see it on this page, they are receiving the same error.

https://api.windy.com/map-forecast/examples/metrics

I've had a windy api implemented for over 2 years now, and suddenly I received the error message, " Leaflet library is missing" and my overlay fails to load. I have changed nothing on my side.

I started to troubleshoot and I was getting nowhere, so I followed the hello world tutorial located here.
https://github.com/windycom/API/tree/master/hello-world

And I get the same error. So what has changed?