ArcGIS Online and Portal for ArcGIS applications allow labelling features using 'Create labels' tool in layer's dropdown menu in web map interface. However, setting label visibility range feature is not available right now. As a result, if labels are enabled for layers in a map, labels will display in all scales cluttering the maps/apps. Therefore, it would be great see 'Set Visibility Range' option available under 'Create Label' tool.
This idea has been merged with More options/controls on labeling features on webmaps, such as scale dependency and stacking.