We have completed the checklist feature as a solution for this.
Here is a related thread if you’d like to support or follow the suggestion for assigning checklist items: https://planner.uservoice.com/forums/330525-microsoft-planner-feedback-forum/suggestions/14924745-assign-checklist-item-to-specific-individuals
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Agree with most other comments below. Wont even bother to try using planner until the functionality is developed to support sub tasks or sub plans.