There is no requirement to have a source tag for every tag. E.g. imagery is the same throughout a changeset and should be added to the changeset itself, but not to every individual object. 

If my edit to "source:geometry" made it too short, please feel free to add the year back to the tag. 

It's "source_ref": "Used to link external source of information" 
https://wiki.openstreetmap.org/wiki/Key:source_ref 
I used the key "source_ref:name" - the reference to the source of the 'name' tag.