Cable in hot car? (Not specific to iPhone 15 Pro Max!)

This a bit of a stretch, but most charging cables have microchips in them, typically at one end or the other. Cables, like most electronics are rated for both operating and non-operating temperature ranges.

For the Apple charging cables, those ranges would be something like the following:

• Operating Temperature Range: 0° to 35° C (32° to 95° F)
• Non-Operating Temperature Range: −20° to 45° C (−4° to 113° F)

So, to more directly answer your question, if you live in an area where you have excessive hot days, with the interior of a vehicle significantly exceeding the non-operating limit, it may cause them to get damaged.

I think the cables should be okay. You may want to put them under something so that they're not in direct sunlight. I've left my cables in the car, well always.

modern devices do not overcharge, I leave all my devices on the charger during the night and never had any problems even if the device was 100% charged when I connected it

there are a lot of battery myths on the internet

have

you read

About

the battery and performance of iPhone 11 and later - Apple Support

and

maybe

iPhone

Battery and Performance - Apple Support

and

Batteries

- Maximizing Performance - Apple