On October 1, 2017 Microsoft added the “Exchange Online Inactive Mailbox” to the Microsoft Product Terms as a new offering and on November 1, 2017 it was quietly removed from the Product Terms. This was highly unusual to say the least. Up until this time there was no charge for inactive O365 mailboxes, provided you followed Microsoft’s guidelines.
In all my years, I have never seen a new product be released to only be immediately pulled a month later. What I have heard is Microsoft pre-announced the new offering at their 2017 Ignite conference and the customer feedback was less than enthusiastic. Pulling a product back is not trivial and I am sure there was some very spirited dialogue in the Microsoft intenal meetings after the Ignite feedback.
I do not think we have seen the last of the Exchange Online Inactive Mailbox. I am not sure when to expect it back, but I believe it will be back one day.