Good news, this feature is partially done. :)
Planner tasks can now be copied to other plans. To copy a task, click on the “…” menu on the task. From there, you can select a different plan to copy the task to.
-WarrenMartijn van der Hoeden commented
So, you can still not copy a Template of Tasks from a Plan to another Plan in a different Team? We create Teams by Project, also because the user authorization is by Team, so this functionality is NEEDED otherwise we need to copy a Plan of 20 Buckets and 30 Tasks manually?