I can only speculate, but having been using kanboard and developing plugins for awhile now, I would speculate that “advanced” goes against the “simplicity”, so I’d be surprised if this feature was implemented as an “advanced” feature. Take that with a grain of salt though, I have nothing to do with Kanboard. I’ve also noticed, that features like that come from contributors, and it is highly possible that someone might take that on.
As for simple way to do what you want, absolutely. whether in the core or in a plugin. I would suggest a plugin, keeps the core intact, and if you ever need to upgrade, you won’t have to re-edit. Off the top of my head, you would simply override the table_column template, and change the condition before it writes the
<?php if (! empty($column['column_nb_tasks'])): ?>
<span title="<?= t('Total number of tasks in this column across all swimlanes') ?>" class="board-column-header-task-count">
in all the instances it’s showing the full swimlane counts.
Once you figure out the appropriate conditions, you can hook a toggle switch anywhere you want, and in the initialize(), conditionally turn the override on/off based on the toggle.