My almost 3 year old phone also turned itself off and is unresponsive today. It was on, just chilling on my couch and charging, no unusual heat. Last time I was using it the battery was around 50% charged.
When I plug in the charger cable (tried 2 cables and 2 chargers), the LED is flashing red faster than a notification would. As described here (with no solution), here (no solution) and here with a solution related to oxidation.
My phone didn’t experience big temperature changes or moisture in the past days, unless carrying it in my back pocket counts. It endured several drops since I own it (only had to replace the display module once). I haven’t dropped it today.
I removed the display module to check the connections and saw that the rubber over the backside of the on button was damaged (which explains why the button was not responding well in the past months). I put a little rubber piece between the contact and the button hoping that would fix my problem but it didn’t help.
I guess I will take it apart completely following the iFixit teardown and then contact support if that didn’t help.