A UK-based leak revealed a plan to require Office 365 inactive mailboxes to have licenses. Storage costs, but because Microsoft told people to use inactive mailboxes for purposes such as keeping content for ex-employees, it wasn’t a good idea to suddenly demand licenses. Good sense prevailed and the plan isn’t going ahead.
Inactive mailboxes are only available inside Exchange Online and don’t exist outside Office 365. Litigation or in-place holds keep inactive but soft-deleted mailboxes in a state in which their data is easily accessible for compliance and recordkeeping purposes.