List Litigation Hold & eDiscovery Hold For a List of Inactive Mailboxes

Step 1 in this article shows you how to collect/report on this data, the only difference when it comes to inactive mailboxes is that you have to include the corresponding switch. https://learn.microsoft.com/en-us/purview/ediscovery-delete-items-in-the-recoverable-items-folder-of-mailboxes-on-hold?view=o365-worldwide#step-1-collect-information-about-the-mailbox

Another option is to use the Invoke-HoldRemovalAction cmdlet, which can be used to both report on or remove specific holds: https://www.michev.info/blog/post/5792/how-to-use-the-invoke-holdremovalaction-cmdlet-to-remove-legacy-and-orphaned-holds-in-microsoft-365