As of GraphQL Admin API version 2023-04, you can now use the Customer Merge API to combine two separate customer profiles with certain non-blocking criteria. You can use the new mutations and queries to:
Additionally, you can check whether a customer can be merged with another customer using the new Customer.mergeable field. This field is also available on the CustomerSegmentMember API CustomerSegmentMember.
You can learn more about merging customer profiles on Shopify Help Center.