We've had two small trailers with front windows
and rock shield covers.
Our Sunlight had a permanently mounted rock shield with adjustable arms. Our Scamp
has a removable rock shield that we made permanent with fixed arms. Both were fiberglass.
Removable rock shields, whether hard or soft, are a bother to have to store when camped. Their advantage is they don't scratch the window.
we used for year had a fabric cover for the front air conditioner used in Casita
16s. It was 15 years old and was still in good shape un-effected by sun and sand, though I expect covering a window with fabric would eventually result in scratches.
Our CRV came with a cloth lined 'bra' to protect the front of the vehicle. It did not take long for the inside cloth to become loaded with dust and what ever.
I think the fact that so many trailers with front windows
have shields indicates the need, at least on some tougher roads.