It's not the macOS 26, its the option is not available on Apple Silicon machines. I think it is always enabled.

https://support.apple.com/en-mn/guide/mac-help/mh40774/15.0/...

Turn Power Nap on or off for a Mac desktop computer On your Mac, choose Apple menu > System Settings, then click Energy in the sidebar. (You may need to scroll down.) Turn on Enable Power Nap. Note: This option is only available on Intel-based Mac computers.

You can see (and change) the settings via Terminal, 'pmset -g' will show the current options.

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)