Ability to hide boards/tasks from Teammembers

Sometimes you have finished tasks that as an administrator or PM need to work on later - e.g. for writing bills and as such you dont want to delete it yet. However, these tasks are not necessary and sometimes unwanted to be seen by the rest of the team. Therefore, it would be nice if you could toggle the task or board to be only visible to the admin of a project (could be a role settings, a task view setting or a board setting)