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Hi E Pangborn,
Thank you for choosing our products. And please accept my apology for the inconvenience caused by the problem.
According to my research and tests, currently, it may not be able to delete/add labels in Planners. As you mentioned we are only supported to edit and add them to a task now. For this information: Flag your tasks with labels - Office 365 (microsoft.com)
I really understand your feelings. So, to address your concerns, I would recommend you vote and leave your comment in this similar User Voice thread: Delete labels – Planner Customer Feedback (uservoice.com) Since that UserVoice forum is monitored by related programing Teams. So, it is a good way to let them know your concerns directly. If this idea is popular, it may be more likely to be implemented in the future.
We highly apricate your understandings and efforts.
Best Regards,
May