In ArcGIS Pro 2.0.1, the legend for ArcGIS Online service layers does not display symbols in layout view when 'Only show features in the map extent' is enabled in the Legend Properties dialog box.
1. Insert the legend into the layout and keep the default legend with all symbols visible.2. Right-click the legend, and click Convert to Graphics.3. Right-click the legend, and click Ungroup.4. Delete the symbols and text in the legend that are not visible in the map extent.5. To group the items back together, hold down the Shiftkey while right-clicking each element in the legend.