Create a Views that displays all unused taxonomy terms
This can be achieved by View relationships and aggregation

1) Add a new view of view type "term"

2) Add Field Taxonomy: Term Name 

3) Add Relationship "Taxonomy term: Content with term"

4) In the View : Advanced section , Enable Aggregation 

5) Add new Filter nid, Aggregation type = Count DISTINCT , Select Relationship = node , Operator= equal to , Value= 0