I believe KA blocks all links from websites outside of the KA domain (with exception to Wikipedia images).

That and I think you can also load javascript from https://cdnjs.com, but as far as I know there aren't any APIs outside of KA's you can access.

cloudflare, cdnjs, jsdelivr, those should all work, I've also seen people load their own repo's from github in html projects.

Ok Thanks 

Can someone send a link of a project that uses cdn from cdnjs in a project

Specifically font awesome cdn

Thanks

This may help you. Sometimes, you can use google :D

https://fontawesome.com/how-to-use/customizing-wordpress/snippets/setup-cdn-webfont