To my surprise, the FAA clause was added in 2006 after the 2004 notice of proposed rulemaking (NPRM) No. 04-11.
Three NTSB recommendations were part of the reason, with the earliest from 1992 (A-92-035).
(...) autopilot failures that can result in changes in attitude at rates that may be imperceptible to the flightcrew (...)
From the NPRM:
This proposed requirement for a visual and auditory warning of autopilot disengagement would be adopted from the current JAR 25.1329(i) and does not exist in the current 14 CFR part 25 (...)
(...) American manufacturers have been providing such a warning, however, as part of compliance with 14 CFR 25.1309, which requires that warning information be provided to alert the crew to unsafe operating conditions. There is a minor difference in the sounding period of the warning provided in American- and European-manufactured airplanes that has resulted from differences in advisory materials and accepted practice, and that difference does affect certification. The harmonization of this rule (and accompanying advisory material) would remove that difference.
As @Sean points out in a comment, the often misattributed 1972 Eastern crash was not an AP disengagement, rather a mode switch to control wheel steering.