Is sign language effective for nonverbal autistic children?

Sometimes. Each child with a diagnosis of autism is truly different from others. Some will respond well and vunderstand signing, others will not. Certainly worth a try.
Joint Attention (JA) precedes verbal or non-verbal communication. Neurotypical tots match their affect to parents' facial expressions & mimic fine-motor games at 9 mos., look for a nod from parents before initiating an act & point for wants at 12 mos., point to share interest & to named pictures in a book at 15 mos.. If kids with ASD "get" a degree of JA from intensive, repetitive therapy, they can then use ASL. .