Collects API now returns only collects associated with custom collections.
API
Effective December 20, 2019
Action required
With the release of the 2020-01 version of the REST Admin API, the Collects API will no longer return products associated with Smart Collections. A new API for returning products inside a collection was released along with a new API for returning a collection by its ID.
Learn more about the changes and how to migrate to the new API in the Shopify Help Center.