I used to use a 35mm SLR (Topcon Auto 100) with the leaf shutter in the body 40
years ago! The problem is that the shutter can only be a certain size by
default, and putting it at the focal plane is problematic for that reason. In
my camera, the leaf shutter was where the lens mounted to the body, keeping it
somewhat smaller than at the focal plane, but that limits the rear element
size, and therefor the ultimate speed of the lens.

--Wilt