Thanks for contacting the Ad Manager API team, this is Yasmin.
I can confirm that it's not possible to retrieve the LineItemSummary, specifically the line item's status, in reports. Kindly note that reports will only return rows where performance has been recorded. If you want to get the complete details of a specific line item, you will need to make a separate API request to a getLineItemsByStatement method by passing the LINE_ITEM_ID from your report.
On the other hand, you can check it's deliveryData or deliveryIndicator fields to know more information about it's delivery status depending on if a line item has an absolute or percentage based goal. Please note that there's no field that explicitly states if the line item is under delivering or not. As you already know, you may instead compare the deliveryIndicator's expectedDeliveryPercentage and the actualDeliveryPercentage to get this information. With that being said, you may check the LINE_ITEM_DELIVERY_INDICATOR and LINE_ITEM_DELIVERY_PACING dimension attributes if one of these is what you are looking for.
Please note that the Ad Manager API only tries to reflect the behavior and available fields in the Ad Manager web interface. Let us know if you still have any further clarification.