I mean, those are the same people who say “we had warm days during summer back then, too”

There is no general legal framework for this. There have been decisions in court to allow the withholding of rent due to heat, or to force the landlord to improve the building. However, there have also been cases where the court ruled that the heat is part of “general risk of living”. It all depends on the exact situation and the decision of a judge, because most landlords will probably tell you to kick rocks when you contact them about this.