Inactive mailboxes in Exchange Online are a vital part of the Microsoft Purview Data Lifecycle Management capabilities. They offer a low/no-cost and versatile way of retaining mailbox data according ...
Updated May 18, 2023
Version 4.0
Blog Post
"critical prerequisite to collect the folder ID values before a mailbox is made inactive" - This is too much of a prerequisite to ask for a large organization, keeping this data for every mailbox to be deleted in hopes that we might have to recover data sometime if a redundant and failure prone activity. Try it on 1TB+ mailboxes, it never completes.
It would be rather easier to allow running folderstaticstics on inactive mailboxes or exporting folderIDs on Content Search Entire mailbox report only exports. So that required folders can be filtered for the actual data export.
"it is impossible to run Get-MailboxFolderStatistics since the cmdlet requires an active mailbox identity" - I don't think this statement is accurate, we can run 'Get-MailboxFolderStatistics -IncludeSoftDeletedRecipients' for inactive mailboxes too and get folderids whenever we need.
Rest of the article is self-explanatory and adds that the folderId based search can be used against Aux archives too, which is good.