As this is a Workflow Manager workflow, you can query
[WFInstanceManagementDB].dbo.Instances table (Workflow Manager database).
SELECT InstanceId,WorkflowStatusDetails FROM Instances WHERE WorkflowStatus = 'Terminated'
The InstanceId can be found via SharePoint for the actual instance of the WF, e.g. if you go to details of the WF instance, the URL will contain the
WorkflowInstanceName value that matches the
InstanceId value from the above query.
WorkflowStatusDetails when the workflow is in a
Terminated status (user manually stopped the workflow), you will have a message similar to:
RequestorId: <Id>. Details: System.Activities.Statements.WorkflowTerminatedException: This workflow was terminated by <email@example.com>
This isn't as straightforward as I think you were hoping for, but the activity is recorded, at the very least.