Currently, there is no way to power off the device without letting the battery drain. Holding down the button triggers a reset. When putting Playdate into long-term storage, it would be preferred to have a way to properly shut it down.
I'm not sure what you mean by the button triggering a reset. Holding the sleep button on the Playdate main menu for ~10 seconds should effectively turn it "off", and the screen should be completely blank.
Unfortunately, I think that prospect is effectively impossible because there is no true passive switch to turn on the device...the momentary button needs to be be polled by the hardware to wake up, making some current drain always necessary.
I had a chance to get back to this, and I'm not crazy: the device truly does reboot automatically when holding in the power button for about 5 seconds and letting go of it.
... while it's plugged in for charging. With the cable detached, the device does appear to power off.