I measured V_VBAT_BKUP pin current like this:
The result shows when VSYS power is On, V_VBAT_BKUP require 1.3uA@2.5V.
If VSYS power is Off, V_VBAT_BKUP require 8.4uA@2.5V.
Next, I will try to find the minimum voltage of RTC working.
However it's difficult to measure correctly, so I expect your information.