You don't really need special data to make GPT-3.5, 4 etc sound as human as to be indistinguishable. The default robotic voice/writing is specifically fine-tune

You don't really need special data to make GPT-3.5, 4 etc sound as human as to be indistinguishable. The default robotic voice/writing is specifically fine-tuned in. Even then, the ability isn't gone, just buried behind the default.