My leather skate boots were not "heat moldable", though I did heat mold them to a limited temperature extant. So it might not seem that they were vulnerable to heat. Not so!
But I punched (stretched with pressure, on a boot press or with a ball-and rink pliers) them to fit me in places where they didn't - ankle bones, toes.
Then I left them in a car that got hot in DC summer temperatures, and they lost the stretch. So they had to be re-punched. I think this is fairly likely to happen if you had to have your boots punched. It happened to me more than once. Such punches are often temporary anyway, and heat just accelerates the process of un-stretching.
Bear in mind that the DC area has pretty hot, muggy summers. But my car is white, and is therefore very roughly 15 degrees or so cooler than dark colored cars on summer days.
Also, I have seen boots that had basically rotted out from being left in the trunk of a car, after a few months. So high humidity can be a problem too. Of course, it is possible that the person had fairly sweaty feet, and that the boots just never got a chance to dry out while in the trunk.
To some of you maybe those of us who won't leave boots in the car in the summer, it may seem we are being over-cautious. But to those who have had problems, or seen problems, it seems an unnecessary risk.
Maybe you will never have a problem. But, then again, you may. It's your choice.