As of May 20th, 2025 apps will be permitted to complete refunds to store credit when store credit was not the original payment method. With this update you can issue store credit to customers as a refund and they can later spend that store credit at checkout. A customer's store credit is visible on their customer accounts profile page and it can be queried via the GraphQL Admin API.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/apps-now-permitted-to-issue-refunds-to-store-credit