I cannot speak on official end date, but here is a blog that talks about migrating to the new map viewer: https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/qa-on-map-viewer/
Here is the latest map viewer update blog: https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/whats-new-in-map-viewer-september-20...
All the information that I have seen is that the classic version will be available till the new one reaches functional parity:
".....Map Viewer Classic will be available until the new Map Viewer reaches functional parity."
Hope this helps!
In case anyone is still looking for it, see Deprecation: Map Viewer Classic in ArcGIS Online Deprecation (esri.com)