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.
An error occurred while saving the commentEllya Gold commented
Moving Tasks between M365 Groups (ie different Teams) won't work because if you've assigned a task to someone Team A, that person may not be a member of the target plan in Team B.
So what happens then? Strip all the task assignees or add them automatically to Team B as members? What if you're an owner of Team A but only a member of Team B? You won't be able to add the task assignee to Team B....it will get rather messy!