Inactive mailboxes have been available in Exchange Online since 2015. A new inactive mailboxes listing is available in the Microsoft 365 compliance center. The GUI isn’t very functional, but perhaps it’s a starting point for some enhanced management capabilities for inactive mailboxes. We’ve only been waiting six years…
https://office365itpros.com/2021/09/23/inactive-mailboxes-microsoft-365-compliance-center/
