With the April 2021 update of ArcGIS Online, the new Map Viewer is out of beta and available for general use by all customers.
For the latest information and detailed instructions on using the new Map Viewer in ArcGIS Online, refer to this blog post and the new Map Viewer documentation.
You can also follow the Get started with Map Viewer lesson to give the new interface and functionality a try.
We'd love to hear your thoughts. Share your feedback here.
P.S. If you're looking for user guides for Map Viewer Beta in ArcGIS Enterprise, you can find them in this post.