Using workspace API you can list out all the notebooks for a given user.

The API response will tell you if the objects under the path is a folder or a notebook.

If it's a folder then you can add it to the path and get notebooks within the folder.

Put all of that in an excel or something and ask your team members if they need a notebook or not.

GET https://<databricks-host-name>/api/2.0/workspace/list
 
Body:
 
{ "path": "/Users/<username>" }

Refer to this documentation for more details.

Also, for a period of 'x' months archive them all in a github repo, in case someone needs access to notebooks later.

Going ahead, add sufficient logs in the notebook or a mechanism to record execution time.

It could be as simple as an insert statement at top cell that inserts a row in a table default.notebook-run with values notebook-name and timestamp, every time a notebook runs.