To fetch only the subscribers from the customer API, you can make use of the customers search endpoint and add the query parameter to filter customers by their acceptance of marketing. Here is an example endpoint that will return all customers who have accepted marketing:
This will return all customers who have accepted marketing, which should give you the list of subscribers that you are looking for. Note that you’ll need to replace mydomain with your actual Shopify domain. I hope this helps! Let me know if you have any other questions.