What temperature of water will kill yeast?

There are multiple temperature points per yeast family might be of Interest:

• The growth limit temperature, see this paper. It indicates as high as 45°C for some and 20°C for others.
• 'Injury' temperature (for bread and brew generally thought be around 120°F)
• Death temperature.

Keep in mind for bread: Bread Yeast at higher temperatures produces off-flavours that may be undesirable to your goal. If you are trying to speed up your dough, you may want to add the yeast to water with some food and let develop for a while before mixing in the flour.

Different strains of yeast have varying tolerance temperature, on both the high side and the low end. As a 'general rule' 110°F is a safe high temperature.

Yeast is happiest at around body temperature - 37°C. The higher you get, the more damaging it will be to the yeast. 30 or 40°C would be fine, but 50°C probably won't (though some yeast might survive). 60 or 70 would definitely kill the yeast.

But for bread dough there's no reason to use a high temperature. I'd mix the water from the hot and cold taps to get something that feels comfortably warm when you put your hand in it, and you should be fine.

I don't proof the yeast either; it goes on top of the flour, dry. I routinely heat the water to 135-140°F and combine it with salt, sugar, dry milk, and oil (this mixing lowers the temp about 5 degrees). The mixture is then poured on top of the yeast & flour, and mixing begins. I've done it this way with both active dry and instant yeast.

I know this doesn't directly answer your question. But I just wanted to point out that yeast can apparently survive brief contact with 130-135°F water.