Muslin (which is a thin open weave cotton based material), like a lot of other things in the upholstery trade, has kind of gone by the wayside. I personally don't use anything (like cotton or burlap) that is not synthetic. The synthetics just make more sense, especially in car seats that sit out in your garage and can be the target of mice for nesting materials. They don't mess around with the synthetic stuff, other than fiberglass insulation which they can pull apart pretty easily.
I replaced a headliner in a '49 Packard, and the mice had filled up the area from the middle of the headliner to the passenger side of the car from front to back with cotton and insulation they had scavenged from the seats.
No one lives forever, the trick is creating something that will.