I seem to have missed the latest update for workflow manager update CU5. Microsoft says to check at https://blogs.msdn.microsoft.com/workflowteam/ for updates to workflow manager. This page hasn't had an entry since 2017.
Keep in mind WFM is more or less a dead-end product, so lack of updates would be expected. There is one additional CU, CU5, which is available from Description of the Cumulative Update 5 for Workflow Manager 1.0.