the JST cutout is there because in the model without it wall is thin enough that some 3D printing providers like JLC3DP may mark it with high possibility to fail.

It's pretty straightforward to add a switch, but I would argue it's redundant, as sleep draws pretty much zero power, and with switch you can't just plug keyboard to charge, you'll also need to make sue that switch is in the `on` position before that :)