At the moment slot.drop_on_stop works only if the Debezium Postgres Connector is shut down gracefully. With Postgres 10 it is possible to mark replication slots as temporary. The Postgres documentation states: "Temporary slots are not saved to disk and are automatically dropped on error or when the session has finished."

So temporary replication slots make it possible that slot.drop_on_stop works even if the connector is shut down ungracefully.