The latest rumor out of South Korea regarding the iPhone 8 says that Apple has ordered all plastic OLED displays from their supplier, Samsung.
http://www.notebookcheck.net/iPhone-8-rumors-Plastic-not-glass.187990.0.html
I don't think plastic OLED refers to the external design but rather the material the flexible display is made of:
QuoteThe plastic part in a Plastic OLED display refers to the substrate, meaning the base layer the display is built on.
https://www.macobserver.com/analysis/plastic-olediphone/