As DITA promotes reuse through keydef and keyref elements. Tridion Docs supports the same and lists down them as references under the map object. However it doesn't list down where the key definitions are used.
It is better to improve the Where used report to allow references of the keydef as well to make it more robust.
Note: I understand keydef are not objects in Tridion Docs, however by assigning an element ID it can be achieved.