When I talk about leakage, I mean out of the outer shell, not inside the battery itself.9V batteries can leak but since some have 6 AAAA cells and an outer shell the leaks can be unseen somewhat sealed away. I've had plenty of 9V batteries leaking. Most of them were in devices that I inherited that I forgot to check the batteries in them. I think some 9V batteries have prismatic cells also that may be less likely to leak perhaps?
I think it is this double layer that makes them so resistant to leakage, (at least in my experience).
But I know they can leak, even out of the outer shell, but honestly I really can't remember that ever has happened to me.
This is why 9 volt batteries are the only alkalines I still dare to buy and use in my electronics.