Yes, some koryū have been lost over time due to factors such as lack of knowledge transmission, historical conflicts, or cultural changes. To try to recover them, researchers gather information from historical sources, ancient manuscripts, and testimonies from practitioners of related martial arts. Additionally, some experts attempt to reconstruct these lost traditions by extrapolating techniques and principles from similar schools that still exist. However, reconstruction can be challenging due to the lack of complete and accurate information. Although not all lost koryū can be fully recovered, these efforts help preserve and reconnect with the historical roots of Japanese martial arts.