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.