Hello Ben,
Map Viewer Beta is available as a separate installation at 10.8.1, and should be available to download from your MyEsri page.
Thanks,
Calvin
The response that I received from Esri was the production version of the new Map Viewer will be available as part of ArcGIS Enterprise (AGE) 10.9.1. AGE 10.9 is shipping with and locked to the December 2020 version of the Map Viewer Beta and AGE 10.8.1 has the March 2021 version of the Map Viewer Beta available as a separate install.
Here is a link to the post about the production version of the new Map Viewer being available at AGE 10.9.1: https://community.esri.com/t5/arcgis-enterprise-questions/did-the-new-map-viewer-and-dashboard-apps-...