[rovolo]

You're correct about it being mandatory:

> At the left front designated seating position (driver's position), a warning system that activates a continuous or intermittent audible signal for a period of not less than 4 seconds and not more than 8 seconds and that activates a continuous or flashing warning light visible to the driver for not less than 60 seconds (beginning when the vehicle ignition switch is moved to the “on” or the “start” position) when condition (A) exists simultaneously with condition (B) ...

> (A) The vehicle's ignition switch is moved to the “on” position or to the “start” position.

> (B) The driver's automatic belt is not in use, as determined by the belt latch mechanism not being fastened, ...

https://www.ecfr.gov/current/title-49/part-571/section-571.2...