We're deprecating the old "syncingTo" field in 4.0. We should remove references to this field in our docs, and replace them with references to "syncSourceHost"
The 'syncingTo' field in replSetGetStatus output is ambiguously named. To make it clear what direction the replication is going in, we will add a new field "syncSourceHost" with the same data as the existing "syncingTo". In a later version we will remove "syncingTo".