Microsoft would like Office 365 tenants to use Microsoft 365 retention policies instead of Exchange Online mailbox retention policies. Their stance is reasonable because Microsoft’s engineering effort is focused on workload-agnostic retention across the Microsoft 365 ecosystem. However, mailbox retention policies continue to offer some advantages that aren’t available in the Microsoft 365 equivalent. And they’re cheaper too because they don’t need Office 365 E3 or E5 licenses.
https://practical365.com/exchange-online-retention-m365-retention/
