At first I thought this was just a typo - and it should be unlicensed mailboxes, however this is not the case since unlicensed users do not have a mailbox, but shared mailboxes do actually contain mail data and/or mailboxes (however they do not have a user id or login).

This is very frustrating because as a termination process we use the convert to shared mailbox as a way to preserve the user's inbox, without consuming a license - but would like to purge this data out after so many days...

I know if you run the powershell command get-mailbox upn |Fl - there is a value called:

And one called 

IsInactiveMailbox                      : False

And 

SKUAssigned                            : True 

I tried to work with a support person yesterday in regards to this, and they were trying to tell me it is operating correctly - and it is not... unlicensed users do not have mailboxes and if you just rename it to say unlicensed mailboxes then it would not include users.