The nav2_voxel_grid package contains the VoxelGrid used by the Voxel Layer inside of nav2_costmap_2d. The voxel grid itself is simply a 2D char pointer array of the map size with bit locations corresponding to voxel values (free, unknown, occupied , etc).
It is branched out as a separate package for use in other applications where a dense voxel grid representation may be useful. It also contains implementations of 3D raycasting.
This package is a direct port to ROS2 for use in the voxel layer.