It says in post
Any device that implements the BLE Heart Rate Profile. It’s an open standard supported by various fitness trackers or health monitors. If you happen to have such a device at home, you can try to connect it with our app, and hopefully, it will work.
But this feature seems to be dropped because it only seems to import from devices now that report HRV themselves, such as my Garmin.
But it could be implemented for any heart rate reporting device with rmssd
The root mean square of successive differences between normal heartbeats (RMSSD) is obtained by first calculating each successive time difference between heartbeats in ms. Then, each of the values is squared and the result is averaged before the square root of the total is obtained.