Will the Map Viewer add on for ArcGIS Enterprise 10.8.1 be updated with the version that comes with ArcGIS Enterprise 10.9.1?
If you mean the previous Map Viewer beta, it will be included in the Portal 10.9.1 installer; https://enterprise.arcgis.com/en/portal/latest/use/get-started-with-mv.htm
If you are asking about having the new Map Viewer functionality from 10.9.1 ported back to 10.8.1, I would hazard a guess that's not currently scheduled to happen.
That is what I'm asking about. I didn't think it would be, but if it was possible, it would be good if it was ported back to 10.8.1.
Upgrading versions in a site that has multiple environments, with a change board, with many teams coordinating testing, etc, is quite an overhead so was hoping to get the release version of map viewer with 10.8.1.