Toybox.ActivityMonitor.Info.isSleepMode was deprecated after system 4.
But new watches, such as the FR970, are still coming out with a sleep mode, including a manual toggle (The moon with zzz above it, not the same as the DND toggle). So there should be some way in the SDK to know if that mode is turned on. It doesn't have to be the same system as it used to be.
On some of Garmin's built-in watch faces, there is that same moon icon to indicate sleep mode is on, so they know people want to see that information, but it's not available for developers designing their own watch face?
On old forum posts, there is a workaround suggested to use the sleepTime and wakeTime variables to check if sleep mode is on, but that doesn't account for the manual toggle.