Did any scholars say that the niqab is a custom or specific to the wives of the Prophet?

None of the scholars have ever said that the niqab or hijab are customs. They differed on the obligation for a free woman to cover her face and hands, whether it is considered awrah (private parts that must be covered) or not. Some made it obligatory, while others deemed it recommendable. In any case, it is an act of worship. As for the Mothers of the Believers, scholars unanimously agreed on the obligation for them to cover their entire bodies, and some even held the view that it was obligatory for them to conceal their very figures.