I have no desire to track my heart rate 24/7, so I leave it turned off except during runs and bike rides.
I was a former owner of a Fenix 5 Plus, and it would still give me sleep tracking data (based on movement) with the heart rate sensor off.
I recently purchased a Fenix 5s that is running v9.0, and with the heart rate sensor turned off, I don't get any sleep tracking data .. however my sleep/wake times due appear in the web version of Garmin Connect, but no basic sleep stage data.
At what point did this change? I sold my Fenix 5 Plus last October.