You can try the impromptu DIY version of such a device by just setting any android-based ereader in front of your laptop display and use VNC/RDP or similar to mirror your screen on the eink reader.

I personally went a step further and used my boox nova 3 and a BT keyboard [1] directly to ssh into my workstation for a while.

Though I agree, an eink based laptop in a X1 or MBA form factor would be amazing, just for the battery life alone.

[1] Logitech K480, comes with integrated tablet holder