Custom Objects in Custom Fields and Attributes is now supported.

Reference Type Custom Fields of type key-value-document are now supported. This enables you to import Custom Objects in Custom Fields. Custom Fields of this type can be imported for every resource for which the platform supports this type of Custom Fields. Attributes (of ProductVariant) of key-value-document Reference Type are also supported. Please see CustomObjectKeyReference for more information.

For example, value of Reference Attribute can now be a Custom Object

{
  "type": "reference",
  "name": "product-custom-object-ref-attribute",
  "value": {
    "key": "custom-object-key",
    "container": "custom-object-container",
    "typeId": "key-value-document"
  }
}