If you read the corresponding feature request it actually seems to be pretty easy. They were just arguing about implementation details.

Windows has supposedly also done this for a while now.

Of course it depends on hardware capabilities. But I guess it’s common enough now.

Edit: This is actually trivial to implement with the rtcwake command. Usually available in linux-utils or similar packages. rtcwake --list-modes shows you what modes your system supports.

Edit 2: Like, this is beyond trivial. Just echo the desired waketime as a Unix timestamp to /sys/class/rtc/*/wakealarm and you’re set. Doesn’t matter if the system is just sleeping or turned off entirely.

Edit 3: But waking from power down seems not to be supported on the Steam Deck. Only from suspend.