This circuit is the basic principle on which my Pi power-off system operates:
The main point being that GPIO14 (Tx) goes continuously low when the Pi is powered down.
When it’s high it’s pulsing, but the power-off system uses programmed hysteresis (a delay!), and this simple indicator doesn’t need nowt! Simply due to the fact that the pulsing is too fast for the human eye to notice.
I use this circuit for headless Pi devices to confirm that they’re ready to have the power pulled, as being a dad I’m so often distracted!!