This change was implemented starting with 2020-01, more details in our API blog here. The reason for the change is that we’re planning to move away from collects altogether, which will allow products in collections to be queried with the new endpoint /collection/{{ collection_id }}/products.json. 2020-01 introduced this change for smart collections, and you can expect to see further changes to the collection resource in the future. Keep an eye on the changelog for those announcements.
Just got clarification that this functionality actually is implemented, you can get all smart_collections a product belongs to using the smart collections endpoint with a query string for product_id:
GET /admin/api/{{ api_version}}/smart_collections.json?product_id=1234567890
Thanks for bringing this up, we’ll look in to why this isn’t in the documentation.