I used to think STM8L would cost less energy than ATTINY. Today I got some time to view the datasheet for these two MCU.
First comes the STM8L datasheet:
If we just use low speed oscillator (LSI) in the STM8L, it would only cost 0.9uA when all other components has shutdown.
How about ATTINY? Then comes the datasheet:
Since we need to enable watch dog timer (WDT) to wakeup, the ATTINY cost typical 4uA in low power mode.
Seems my memory is still correct 
STM8L is the winner.