In the meantime, you can avoid the deprecated warning by not setting any properties on the line_item object other than variant_id and price.
Here is an example of a valid request to create an order that sets a customer and sets a line item’s quantity to 1:
{
"customer_id": CUSTOMER_ID_REDACTED,
"line_items": [
{
"variant_id": VARIANT_ID_REDACTED,
"quantity": 1
}
]
}
This request will create an order with the specified customer and line item. It will not throw a deprecated warning.