Very pleased to announce that Developer Edition is out of beta. You can download it from the ArcGIS Developers website. See the blog post.
As you may notice, the widget code is open source in the final release. And the great effort has been put into the SDK documentation. Especially, information about classes, methods, and properties in the API Reference are pulled dynamically from the source code of Experience Builder. So you can access the latest API information when a new release arrives.
Again, your input here and in Early Adopt Community help shape this product. For that, we thank you! Together, we can make a difference.
Cheers,