On Apple Silicon it's not available under the battery settings, but you can still set it.
> sudo pmset -a powernap 1
-a is an option to set it for battery and plugged-in. If you want only either you can do -b for battery, -c for charger
You can also check the settings with:
> pmset -g
you can also use pmset to programmatically enable/disable low power mode (which i do when the battery gets too hot for my liking even before thermal throttling kicks in)