For me, it's less about the actual temperature and more about the humidity. Here it was 91 with a heat index of 101 and a dew point of 76. That tells me that humidity is very high and, in my mind, it's not safe to work a horse hard. In areas where it's a drier heat, it may be easier to work a horse, but I don't know since I live in Ohio. I don't typically ride if it's above 90 with a humidity of 70% or more. Once it's above 70% and hot, it becomes unsafe for me to ride, and I don't want to put a horse through that. That's my thought on it.
The wind of heaven is that which blows between a horse's ears. ~Arabian Proverb