Are there any real world examples of encoding data in EM polarization?
A receiver connected to a simple receiving antenna cannot distinguish between a plain carrier of polarization A and the same carrier transmitted with a different polarization and amplitude - they could look the same to the receiver.
If there was something that could re-polarize the transmission as a means of modulating the carrier, the receiver would just detect it as amplitude modulation.
Simple transmit antennas (and probably some quite complex ones too) can't be electrically made to change their polarization.
Signal reflections from objects can reorient polarization and this could muddle things up.