How to delete all inactive plans created by others

I am a Global Administrator for a Microsoft 365 tenant. Some of the tenant's users have created plans over the years. Some of those users have also left the organization and have had their user accounts removed. However, the Microsoft 365 groups are still being utilized for other apps like SharePoint, Teams and Power BI.

I want to delete all the inactive plans that exist in some Microsoft 365 groups. Is there a way for me to do this via PowerShell or as a Global Administrator, access all the plans created by all users and choose which plans to delete? 

As mentioned above, the Microsoft 365 groups are still being used, so I can't delete the Microsoft 365 group.