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!
Would like to know this also. Thanks.
In case anyone is still looking for it, see Deprecation: Map Viewer Classic in ArcGIS Online Deprecation (esri.com)
Map Viewer Classic in ArcGIS Online will be deprecated with the ArcGIS Online June 2023 update (planned for June 14, 2023) and is expected to be retired in December 2025. This deprecation is consistent with the deprecation of the ArcGIS API 3.x for JavaScript on which Map Viewer Classic is built.