The temperature that is too hot outside for houseplants are temperatures above 80 F, especially if the houseplants are in confined spaces.
Temperatures that get above 90 F for houseplants can cause heat stress and is considered a heat stress baseline for most plants including houseplants.
Any prolonged exposure to the high temperatures for the houseplants and some other plants can cause leaf wilting, weakened plant immune systems and decreased flower production.
Some plants can tolerate higher temperatures at least for a short while but it's best to keep the houseplants and other plants out of extreme heat if possible.
If you have outdoor plants and they are out in extreme heat it's best to allow them to be in some shade during extreme heat and ensure they get enough water but not too much water.
For some houseplants even temperatures that are consistently above 75 F can be considered too hot or too warm.
And prolonged exposure to temperatures above 85 F to 90 F or higher can cause heat stress in the houseplants and other plants.
You also don't want the houseplants to get too cold and the temperature of the house should stay above 50 F.
Also if you want to place the houseplants outside for awhile you should make sure the outdoor temperature stays consistently above 50 F.
And if nighttime temperatures are forecast to fall below 50 F you should bring the houseplants back in for the night and then bring then outside again if you want to do so when the temperatures rise again.