I understand that this is valid for stationary solution which is what an astronomical black hole would be most of the time. What I don't get is why do you assume that this would violate causality. In GR the idea that changes must “instantaneously update” large regions of spacetime due to supermassive black holes ignores relativistic effects like time dilation. From the perspective of an external observer, as an object falls into a black hole, time appears to slow down near the event horizon. The infalling object’s influence on the black hole’s external field (e.g., its charge or mass) is perceived gradually by distant observers.

For supermassive black holes, this process might seem slow due to the massive scale of spacetime curvature near the event horizon, but there is no requirement for instantaneous changes. In fact, relativistic causality ensures that no information can propagate faster than light, so updates to the black hole’s charge, mass, or angular momentum are constrained by the speed at which signals (gravitational or electromagnetic) can travel.