Similar to CHMSL regs. Cars were required in 86, trucks not until the 90s. 94 or 95 I believe.My 1999 2500 doesn't have one. It was based on GVWR. Equal to or less than 8500 lbs. GVWR got airbags. Greater than 8500 lbs. GVWR were not required, and so GM did not install them at the factory.
it was based on federal law. This is probably the law:
https://www.law.cornell.edu/cfr/text/49/571.208
I'm not really motivated to do more research into it. I do know one of the factors was GVWR though.