Take what I am about to type with a grain of salt for a few reasons; 1) this is anecdotal, gathered from my own experience with both my family's cars and customer's cars, 2) I haven't turned wrenches for a living in close to fifteen years and even then I mostly worked on British cars from the 50s-70s and American cars from the 60s and 70s, 3) my newest car is now thirty years old and the other car I regularly drive is sixty-four years old.
I have lived in both hot (110+ in the summer) and cold (-20 in the winter) places and I have seen many batteries killed by the cold and none I can think of killed by heat. This seems to hold even in new cars, I just got done replacing the battery in a 2012 Ford Expedition after a week of temps dropping down to single digits over night and not hitting 20 during the day. Yes, I have seen dead batteries in the summer, but those are usually old batteries, like the one in my Miata which I had to change this past year but it was also over five years old. The one I just changed in this Ford was only two years old.
Now, having said that, I do think that circumstances in cars have changed. Even in my newest car the battery doesn't have to deal with the extreme heat produced under the hood of modern cars, which are designed to run at very high temps, over 200 degrees. The operating temperatures of these cars creates a much hotter conditions under the hood than even the hottest day in Death Valley can create for an old car. My old pickup has 160 degree thermostat a large radiator, and an engine compartment that makes the V8 under the hood look tiny. By comparison in my son's 2019 Dodge, you can't even see the ground when looking under the hood because the engine, all the plumbing, all the computers, etc are stuffed into a bay that's half the size. We had to replace the radiator, water pump, etc in his pickup last August and when we were done and it was running I hit it with my laser thermometer, and the engine was close to 215 degrees, he googled it and that turns out to be the running temp. Everything under that hood was hot as a result. Just for fun, I fired up my Binder and let it idle up to temp and hit it, at the thermostat neck it got to 172 degrees and under the hood was really only marginally warmer than the ambient temperature.
So, now that I have been super long winded, if the thermal blanket is there to protect the battery from high temperatures (which it may well be there for); it is to protect it from the excessive heat generated by modern cars and not from a hot day. Also, I still see more batteries die in the winter than in the summer. Just my two cents, though with inflation it seems a lot more like a dime's worth. If you feel the need to replace the blanket, I would do it now, because that temp under the hood is only going to be a bit cooler now than it will be in the summer.
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