I wonder what happens, at the same pressure, as the temperature drops. Maybe a new form?
http://www1.lsbu.ac.uk/water/water_phase_diagram.html> Here we use laser-driven shockwaves to simultaneously compress and heat liquid water samples to 100–400 gigapascals and 2,000–3,000 kelvin.
And in the phase diagram that you cited, it stays at ice XVIII over the whole temperature range.