I'm pretty sure my dad's ~2001 saab 9-5 had an indicator on the dashboard as to whether the passenger side airbag was turned on. Of course, I could be misremembering.
It appears that passenger side airbags that automatically disable are not required, but they were explicitly allowed ca 1997 by the NHTSA (as well as allowing a manual on/off switch). So my original point was somewhat mistaken. Of course, cars do have a weight sensor to alert you when the passenger seatbelt isn't fastened, so still, the car knows when you have a passenger.
