Ming-Chi Kuo, an analyst specializing in Apple, says that the company is testing with an electronic ink screen which could be for an outdoor screen, or part of a folding device.
E-ink is already used by Amazon Kindle, but Apple has been researching its use since 201. Now Kuo says that the company is actually testing a color version.
The statement that this technology could be used as an external screen due to its energy savings suggests that Apple could be thinking of a folding smartphone. It also points to the possibility that you are thinking of a tablet device.
Although he doesn’t cite any sources, he seems to be saying this as something happening at Apple right now, and not as speculation.
Ming-Chi Kuo has recently predicted that Apple will release a foldable iPhone in 2023, though he thinks the screen will be high-resolution QHD+ rather than e-Ink.