I see in the new ESRI blog, that related records are now available in the (new) Map Viewer: Blog
BUT, that is only if you are using a hosted feature layer from AGOL??? Really? This does not work with services with stored credentials? So, I decided to test it. Related records aren't available if you use an ArcGIS Server service with stored credentials and added as an item in AGOL.
Will this remain the case?