Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility with mono AM receivers; and PM is basically the integral of FM.
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".