Office 365 plans E3 and E5 provide 100GB user mailboxes and unlimited mailbox archiving ..and then there’s the archiving that’s NOT unlimited. Office 365 plans Business Essentials and Business Premium provide 50GB user mailboxes and 50GB archive mailboxes.
In Microsoft 365, things are a bit different. The same 50GB goes for the Microsoft 365 Business user mailbox, but there’s an unlimited archive mailbox. Microsoft 365 Enterprise has the 100GB limitation on the user mailbox and unlimited on the archive mailbox, too.
“Microsoft 365 Business includes Exchange Online Archiving subscription. Each Exchange Online Archiving subscriber initially receives 100 GB of storage in the archive mailbox. When auto-expanding archiving is turned on, additional storage is automatically added when the 100 GB storage capacity is reached.”
Please bear in mind that archive mailboxes are also searchable.