Good news - I just installed the new firmware R960XXU1AWK4 onto my Galaxy Watch6 Classic, and now the heart rate complication correctly displays the heart rate data!
@amoledwatchfaces confirmed the same at Heart rate and weather as fixed complication providers - #8 by amoledwatchfaces
This proves my point, that it actually was and should have been possible to display this data, that it had nothing to do with Privacy Policies, and that it was in fact a bug.
Great that the devs have finally fixed this!
I noticed that the Sleep complication also now correctly shows its data too, where it wasn’t doing so previously.