Just noticed this new support for customizing map's visible scales: type in the scale you want such as 200 and click the plus sign then you can zoom in beyond the base map.
IT Sr Analyst
GIS Timeline: 1988 - Present
From Ink & Mylar drafting, ArcInfo/UNIX Workstation v6.1.1, COGO, Map Librarian, ArcEdit, ArcPlot on AIX, ArcIMS and AutoCAD v2.6
To current ArcGIS, Portal and AGOL