A long while ago these were put forward:
https://www.digitaltrends.com/mobile/smartphone-e-ink-covers/
but I can't recall that there was ever an actual product.
It seems to me that a case which:
had an e-ink screen which would show when protecting the device (mirroring the display)
which would open in such a way that it could either extend the display, or function as a keyboard/touch pad/stylus digitizer while supporting the cell phone
would be a nice option, esp. if some manufacturer would standardize their form factor over multiple phone generations allowing the investment to be amortized over the lifespan of several phones.