@emilyb: Yes, the segmentation_type=REGIONS is still supported. Please note that the country param is required for this segmentation type. While you’ve included it in your request, it must be specified as a GeoId. To find this ID, use the GET targeting_criteria/locations endpoint. For US, the value is 96683cc9126741d1.
Your params would be: segmentation_type=REGIONS&country=96683cc9126741d1.
Hope this helps!