There's no set temperature that makes it illegal for people to work outside in the heat.
However when the outside temperature gets 100 F or above then it really is too hot for most people to safely work outside.
But you can still be required to work an outside job even in 100 F and above temperatures but if you do need to or have to work out in the heat you should make sure to dress lightly and drink plenty of water through the day.
Take frequent brakes in shade or air conditioning if and when possible.
If you don't have to actually work in the heat then wait until early morning or evening when the heat is not too hot before working outside in the heat.
I used to work an outside job and we were working outside when it was 110 F and I finally could not take it anymore and just quit because I decided that it was either I quit the job and live or I keep at the job and suffer and possibly die.
I decided to quit and now I work an office job where it's cool inside and I prefer my office job instead of the outside job.
I also make more money working in the cool office than I did working harder in the heat.