Ming-Chi Kuo's latest report suggests that Apple is looking to set its upcoming foldable devices apart from other brands by utilizing electronic paper displays. These solutions will be used for the cover / secondary displays due to low energy requirements, but the main foldable displays will continue to be OLED-based.
https://www.notebookcheck.net/Apple-allegedly-testing-electronic-paper-displays-EPD-for-upcoming-foldable-devices.620142.0.html