Many vehicles are switching to indirect measurement (this is just my impression, I haven't exactly been tracking which years/models use what system). It's certainly cheaper, you only need to write the algorithm once. Early systems mostly used sensors. I'm not sure why they used sensors to begin with since it's not like there's been a big improvement in VR sensors (the thing used to track wheel speed) enabling more accurate measurement. I suspect that the unwieldiness of big engineering organizations plays a role.