You're right, AvailabilityManager:HighTemperatureTurnOff is not in the OpenStudio SDK yet. This was an oversight and I've filed an issue for it.

In the near term, you can either:

• Use an EnergyPlus Measure to add this object.
• Use an OpenStudio Measure to write EMS (available in the 1.13.3 developer release, and soon to be 1.14.0 major release) to control it yourself.