The web app is here -- 3D Data Preview (arcgis.com)
The sharing level is set to everyone and all the layers I own are also set to everyone. I can't seem to figure out what has changed in the service that now a user must login in order to view the web app. The web scene used in the web app uses the Terrain 3D ArcGIS Tiled Service Elevation Layer and the World Imagery Tile Layer.