So, I am pretty familiar with the STM32L4xx low power Cortex-M4 MCU.
It has some insanely low power consumption profiles including a 0.65 µA standby mode with RTC (and just 14 µs wakeup time).
This thing is a beast to program (1600+ reference manual to start with and copious app notes). But, I work with this chip for a living.
This current consumption is specified (in the documentation) at 1.8V, so a more typical 3-3.3V actual power supply will likely cause it to consume more current. I am assuming direct battery hookup otherwise the current consumption of an LDO regulator has to be considered.
Still... this is insanely low.
I am looking to play around a bit with some "old" 8-core Parallax Propeller Chips (P8X32A) I have laying around and I read some forum discussion where you could likely get it to consume as little as 7- 10 µA when running just one COG doing not much (maybe as a timer?).
To these jaded ears that sounds like a lot of power, but... honestly... really?
With a couple of 1200 mAh AAA batteries (3V) the P8X32A would run around 11 years. That is greater than the shelf life of AAA batteries.
Most of the current consumption these days aren't from the MCU but from the peripherals and sensors. If I wanted to beacon some temperature measurements via BLE (connection-less -- just as a "tag") maybe every minute, the battery life drops to around 1 year. So, it would be more reasonable to beacon every 10 minutes. Then I could get maybe 5-6 years.
We get lured into thinking too much about how low an MCU can go, when in the world of IoT, it is the RF that is killing us.
Just food for thought.