As of API version 2025-07 you can now narrow the list of fulfillments returned by Order.fulfillments with an optional query argument that targets createdat and/or updatedat fields. The argument uses the same search syntax already familiar from other Admin API endpoints, for example:
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/add-createdupdated-at-time-based-filters-for-an-orders-fulfillments