Are there placebo buttons in regular cars? Why would self driving cars have this as a feature? Especially whoever is paying for the car which would have to pay money for this useless feature.
As far as placebo effects in cars, a lot of newer cars are putting in "sound enhancing" tubes to pipe sound into the cabin from the engine bay to make the car sound more powerful. VW went so far as to actually play engine noises out of the speakers regardless of if the stereo was on.
Well, there are certainly buttons that do things that have little real-world impact on the car. Like on high-end German cars you get seven different levels of ride stiffness etc. that can be selected.
Current cars do have placebos though. Some newer sports cars would play engine sounds thru the speakers to make users feel more power.