Correct me if I'm wrong but such a thing doesn't exist: I want to make AI units not walk through an area by removing it from their pathfinding logic.

It could be created like this:
A new "ModuleAreaRestriction_F" is a module object in Arma 3 that can be placed in the mission editor to define an area where units are restricted from entering or leaving. When the module is activated, it creates a trigger volume around its position that can be customized to fit the mission's needs.

The ModuleAreaRestriction_F has various configurable properties, which can be accessed through its attributes in the editor or via scripting. These properties include:

Activation: Determines when the module should be activated, such as when the mission starts or when a trigger is activated.
Condition: Specifies a condition that must be met for the module to activate.
Area: Defines the size and shape of the restricted area. This can be set to a circular or rectangular shape, with customizable dimensions.
Color: Determines the color of the area restriction zone on the map.
Title: Sets the module's name, which is displayed in the editor and on the map.

Once the ModuleAreaRestriction_F is activated, all units will not consider that area for their pathfinding system to walk through.