If this is set to 2 or higher and the leaf_size is small relative to the density of the point cloud, part of the pointcloud may disappear, which can be a surprise to someone unaware of this parameter. Therefore, I think this default value should be restored.
The default value of min_points_per_voxel is changed in migration (#398).
The default value in melodic was 1. https://github.com/ros-perception/perception_pcl/blob/283496fd921a0b60e9ea1bba45561804226895e8/pcl_ros/cfg/VoxelGrid.cfg#L19
After migration, it was changed to 2, but I could not find any reference to the reason for this. https://github.com/ros-perception/perception_pcl/blob/e8ce25267a6d9d0bb0d80ba17e9c65269a7254aa/pcl_ros/src/pcl_ros/filters/voxel_grid.cpp#L72
If this is set to 2 or higher and the leaf_size is small relative to the density of the point cloud, part of the pointcloud may disappear, which can be a surprise to someone unaware of this parameter. Therefore, I think this default value should be restored.