Hi Chad - today if your Plan has enabled the "Send notifications about
task assignment and task completion to the Plan's conversation feed."
option in "Edit Plan" under the "..." menu, Planner will send emails to
the group mailbox when tasks are completed. We're exploring options
around this space, so let me know if you have any scenarios in mind.
Hey Chad, not the greatest solution, but you do have the option to build a flow since a trigger exists for "When task is completed" So you can technically have whatever you want to happen when a task is completed on a plan. It does require a quick flow to be setup, but it's possible.