Please note that Planner has its own UserVoice site that its team monitors for feature requests. You can find it here: http://planner.uservoice.com/
4,221 votesthinking about it · AdminMicrosoft 365 Groups Feedback (Product Owner, Microsoft Office 365) responded
Thanks to all of you for your feedback on this item. We know that folder creation is still a high priority request from our customers. Please continue to provide your feedback as we assess the priority in relation to other initiatives the Outlook teams have committed. Thank you!
Thanks to everyone for your feedback and votes on this item. We’re working through a couple of options and hope to have more information shortly. It’s a very difficult problem to solve since group membership is consumed across many different aspects of Microsoft 365. Thanks for your patience and keep the feedback coming!