This is true for most of Briggs vertical engines especially for those design for walk behind use. This is to try to prevent internal damage to the engine and the PTO end. I still get them in the shop sometimes with bent crankshafts even with the Briggs aluminum keys.
There is one set exceptions to use of the aluminum key made in 2012 to the 210000, 280000, 310000, and 330000 verticals where there was undue shearing of the key. Briggs changed those engines flywheel key to a steel key PN 796335 and increased the torque spec to 110 ft-lbs.
This was mainly due to a flywheel key-way design problem which Brigg redesign starting with date code 120630*******. The flywheels and crankshafts of these engines are not interchangeable. There was no mention of going back to aluminum key but they may have after the redesign.
References ASPI 82 and ASPI 89
