https://support.workfront.com/hc/en-us/articles/115000165713-API-Version-Release-and-Deprecation-Sch...
According to the page above:
NOTE All Workfront URI calls must specify a version of the API. Integrations that do not specify a version in the URI are automatically routed to the Default Version of the API and will fail once the Default API is removed July 1, 2018.
I think default API should be the latest version of the API. Asking the user to redo all programs and scripts every few months or so to update/test the API version is too much effort specially if there's a lot of automation already built to integrate to Workfront.