In this paper, a comprehensive review of the evolving engineering and testing methodologies for radiation hardness assurance (RHA) in commercial-off-the-shelf (COTS) based space avionics, with a focus on recent trends and future directions is provided. The increasing reliance of space engineering on COTS has prompted a shift in RHA strategies, reflecting both technological advances and the complex radiation environments faced by modern space systems. Emphasis is placed on the interplay between traditional RHA frameworks and the integration of state-of-the-art design principles, fault-tolerant architectures and testing approaches. The review highlights how evolving system requirements and accelerated development cycles have influenced radiation testing practices and risk mitigation techniques. Examples are presented of enhancing reliability under radiation exposure, including reconfigurable systems, agile engineering processes and system-level validation. Emerging applications, including intelligent onboard systems and distributed satellite networks, are discussed with attention to their unique challenges and opportunities in RHA. The review concludes with a perspective on the critical gaps and future needs to advance RHA practices in support of increasingly complex and resource-constrained space missions.