Question

is this sentence essential or nonessential? This theory, which may at first seem like a old wives tale, actually has some basis in fact.

Answer 1

The sentence is nonessential.

Answer 2

The sentence itself really doesn't give us a hint. It can be a very good lead in statement for something that discusses cures currently classified as Old Wive's Tales. If you google the statement it comes back with all kinds of entries some of them actually relevant.

I would say that it can be essential. It depends on the context.