I’ve kept my paddock boots in car year round, gets to about 30C/90F here regularly in the summer. I feel like water damages them more than heat, but I don’t know if they would have held up longer if I wasn’t doing this (or if I was good about cleaning/oiling them, because I basically never did). These were 8-9 year old Blundstones, if that makes a difference; previously I had several pairs of Ariats that I did not keep in car (stayed in barn locker), that tended to last 4-5 years.
The main issue I had is that they can make car smelly, even in the trunk. It was better when I took them out at least once a week to air out and would not leave them in car if they were wet – they go out in the sun to dry. Don’t know about tall boots, but they have stayed in tack trunk in the same heat, though not daily/all summer. Humidity may be more of an issue than heat also?