The usual long distance very high voltage transmission lines only run three wires, with the transformers at each end connected in delta. There is no neutral wire.
The reason for this is it can transmit the most power, with the lowest losses, and with the least amount of copper.
There will definitely be a ground fault interrupt system that will completely disconnect power if one power line becomes suddenly grounded due to some serious fault. But the threshold trip current will be set fairly high to prevent nuisance tripping.
The system is not completely floating, as there will be considerable capacitance to ground, especially around the transmission towers, plus some leakage current to ground at every dirty cracked dust covered insulator.
So the system sort of ends up having a very poorly balanced voltages around ground potential. Any direct physical path to ground from one phase will conduct current to ground through the stray coupling of the other two phases to ground.
So it can kill you for sure, even though it is in theory a fully floating system.