Under the Resting Rate metric in the Health app on the phone, my Series 0 watch doesn't even show up in its Data Sources list. So it's possible this is exclusive to the Series 3, although I don't know if they have improved on the Health sensor on the series 3 to justify this, or if it needs an additional accessory for this to work. Maybe it only works with third party apps.
In the Heartrate app on the Watch, there's a new "Recovery" metric that indicates what the heart rate recovery was after each workout. This shows up after a workout performed, and it's not exclusive to any watch. You just have to perform a workout that logs to the Health app. For me, it shows up after every workout I complete. It seems to calculate the heart rate loss recorded right after the workout and three minutes after.
If you have multiple workouts performed on a single day, the "Recovery" heart rate appears after each workout in the Heartrate app on the watch:
View attachment 717502
See my above post. Resting heart rate != Recovery heart rate.