To effectively facilitate the deletion of every instance of data associated with a deleted user account from all data stores, the most reliable approach is to build a standardized and documented retention program for user data deletion. This program ensures a systematic process for identifying and removing user data across all data stores in the organization, ensuring compliance with data protection principles. By having a documented policy, the organization can maintain consistency and accountability in data deletion processes. This method is recommended by data privacy standards and is elaborated in IAPP's Information Privacy Technologist resources.