The green-LED discrete LM3909 is still flashing, even with its AA NiMH cells burned down to 1.15 V:
If the truth be known, one of the cells is now reverse-charged to 200 mV, so that’s a bit beyond as low as it can go.
The flash period has stretched to 8.7 s:
The circuit boosts the battery by 800 mV to put 1.94 V across the green LED at the start of each flash:
Admittedly, the LED isn’t particularly bright at 2.8 mA:
But it’s still flashing!
Swapping the cells into the LM3909 with a blue LED doesn’t produce any blinking, which is about what the earlier tests showed.