[StarGate]

Found this for Canada laws.
"Each turn signal lamp on a motorcycle manufactured on and
after January 1, 1973, shall have an effective projected luminous area
of not less than 3\1/2\ square inches."

Figured the US would have enacted something earlier than Canada.
Then I found this under US Federal Motor Vehicle Safety Standards.
Standard No. 108 - Lamps, Reflective Devices, and Associated Equipment - Passenger Cars, Multipurpose Passenger Vehicles, Trucks, Buses, Trailers, (except pole trailers and trailer converter dollies), and Motorcycles (Effective 1-1-68 for vehicles 2,032 mrn (80 or more inches) in width and Effective 1-1-69 for all other vehicles)
This standard specifies requirements for original and replacement lamps, reflective devices, and associated equipment. Its purpose is to reduce traffic crashes and deaths and injuries resulting from traffic crashes, by providing adequate illumination of the roadway, and by enhancing the conspicuity of motor vehicles on the public roads so that their presence is perceived and their signals understood, both in daylight and in darkness or other conditions of reduced visibility

And this
Standard No. 123 - Motorcycle Controls and Displays - Motorcycles equipped with handlebars, except for motorcycles that are designed and sold exclusively for use by law enforcement agencies
(Effective 9-1-74)
This standard specifies requirements for the location, operation, identification and illumination of motorcycle controls and displays, and for stands and footrests. Its purpose is to minimize crashes caused by operator error in responding to the motoring environment, by standardizing certain motorcycle controls and displays.

But still haven't found exactly what I was looking for. Based on some of that info I would guess federal law required them sometime around 1970 or a little earlier.