As I understood it, the dashed line is just the 35°C wet bulb temperature line.
I think it’s the “old assumed border of survivability” and don’t know if it is based solely on mathematics or on other experiments as well.
I also don’t know on how many individuals the new line is based and what age group the older people one is.
I recently read somewhere that it’s actually just very few bee species that die after stinging, among them honeybees. They have a barbed stinger that gets stuck while most bees have flat stingers and can sting repeatedly.