in odoo the project.task class has tasks and sub tasks , both are the same the only diffrence is the parent_id if it's empty then it is a task , and if it has a parent_id then it's a subtask. i found that tasks are trackable but subtasks are not, how to make the subtasks trackable too
in the settings go to project then you can find sub-tasks checkbox to unable history for subtasks, then in the formview of any task you'll find a smart button to lead you to the a list view of the task's subtasks , after that just select any subtask and you will se its history.