For the super high capacity demonstrations, 256 QAM and/or probabalistic/geometric shaping is typically used so we get to about 12 bit/s/Hz (accounting for FEC and pilot overheads). Interestingly, data rates are mainly limited by the transceivers (RF amplifiers, DAC/ADC ENOB... is not that great at 25-100GHz, which is required for the 50+Gbaud symbol rates).
