@azonenberg Assuming it's STM32L431xx in UFBGA100, the "bridging" GPIO is actually PC15, which has footnotes in the datasheet indicating that it belongs to the "backup power domain".
So, in case one would really use "Vbat" for the purpose that gave it its name it seems one would be safe.
Bug then again, when using it for a RTC, that GPIO would be consumed by the 32k-crystal 🤷♂️ ...