Users (including this reporter) began noticing this change in the last several weeks, even though an Apple spokesperson says it was added in the first release of iOS 9. However, a bullet point describing this restriction only appeared in the iOS Security Guide on May 12, 2016, according to the guide’s internal PDF timestamp. Apple declined to explain the rationale for this restriction.The previous five passcode requirements are: the device has been turned on or restarted, the device has not been unlocked for 48 hours, the device has received a remote lock command from Find My iPhone, five unsuccessful Touch ID attempts and adding new fingers to Touch ID.
It's unclear why Apple added the restriction and why it chose an eight-hour window, but the rule comes after a judge granted a search warrant forcing a woman to unlock her iPhone with Touch ID. The decision comes as some believe the biometric nature of Touch ID isn't protected by the Fifth Amendment's protection against self-incrimination. Passcodes, however, are considered protected individual privacies.