The Office 365 Planner app doesn't support PowerShell, so the Graph is the only route to Planner data. However, you can combine PowerShell and the Graph to retrieve information about plans and tasks to understand what plans are in active use. Because the Planner API only supports delegated permissions, some thought needs to go into how to access data, but it's straightforward once you understand what's happening.
https://petri.com/working-planner-data-through-graph
