ArcGIS Pro has the ability to repair layer data sources when the data source cannot be found. The broken data source is shown in the Contents pane with an exclamation mark (!):
Selecting the exclamation mark opens the Change data source dialog so that the link to the data can be repaired.
I would like to see similar functionality in the ArcGIS Online Map Viewer that will allow for searching for a missing feature layer and repairing the link to the data source. Subsequent saving of the map would save any changes.