The thing I would be worried about is that when wet, fallen leaves will retain moisture and rot. Chicken poop is about 75% moisture. Even if it seems dry on top, it can be wet and rotting in the layers below. The main thing I would be worried about (aside from the bugs and creepy crawlies) is when it starts rotting, will it impact the chickens' sensitive respiratory system?
If you are to use this as coop bedding, I would definitely completely change it often and probably not use a deep litter method. It would also depend on what type of floor your coop has. If your coop has a ground/dirt floor, it may be great. If you have a wood floor, this bedding may rot the floor out.
Think of going hiking in dense woods in the middle of fall. If you dig down into the leaves, it will always be very moist and the layers will be rotting.
I use dry leaves as large part of my deep litter. (Mix of trees including oaks) I also use grass clippings (I have a neighbor who throws theirs in, we leave ours on the lawn) Pine shavings, weeds, straw...anything that I would compost.
I use leaves and grass clippings as a deep litter on the ground level of my coop. It makes a wonderful mix. The leaves, grass and poo melt right into the ground with no odor. In the loft of my coop, I use shavings. I think you would do well to mix the leaves with shavings if your coop has a standard floor. Keep an eye on it, and let your nose be the judge. If it starts to smell, you'll know that you have a problem.
I also use leaves and grass clippings, but I mix well with pine shavings. Just the plain leaves get too slimy and stick together when wet in my experience. But dry leaves, spread over top of shavings or such, and turned well into deep liter, are just one more component.
I used leaf mulch and straw bales to make a potato bed last year. It worked out well. After I dug the potatoes I set up my portable run around it and let the hens have at it. Within days they had it scattered out and ready to till under. No adverse side effects from it. So I was also thinking of using some of the leaf mulch this fall for deep litter.
I use the deep litter method in my coop/run. I use leaves, pine straw, grass clippings and shavings. I rake up as many bags as I can in the fall to use throughout the year. Saves money on buying shavings only for the litter. The chickens love to scratch through the leaves to see if they can find any "goodies". Gives them something to do and makes the coop smell good.