Christoph put together a soft power button with sense output to power up a Teensy and allow the application to turn itself off the power when necessary.

The project started with list of desired features including: one button to enable power to a circuit, keep the power on as needed, sense the output so that the microcontroller can sense the power button’s state and turn itself off when told to do so, use a single LiFePO4 battery, and use no specialized parts.  Christoph was able to make this happen using a Teensy 3.6, a 5V step up/down regulator, a PCB he designed for the pushbutton, and a few passive parts.

Code for the project can be found on this Hackaday.io project page.