Don't have any links. An old club maker told me that because I kept my bag in my trunk. Heat does a lot of nasty stuff, especially here in Florida. Heat won't affect steel and the epoxy needs 200 degrees I think.
I did a quick search and found some comments about heat but most were about the epoxy. I guess over time the repeated heating could affect the epoxy. I'm still more concerned about the affect of excessive heat on my balls and shaft.