We'll also add this caveat: Barbie is a toy. Doll toys historically have not been anatomically correct. Also, no gendered double standard exists in Barbieland: Ken is as featureless downstairs as his girlfriend. Barbie crotch in dolls is gender-neutral.
2nd century Roman doll, displayed at the Centrale Montemartini
What we're talking about today when we discuss Barbie crotch in art is a trend among European artists to depict the vulva as featurelessly and hairlessly as a Barbie doll.
After the Bath by Pierre Carrier-Belleuse, 1896, Zoullas collection