need notification in retention policy when it applying to Exchange online mailboxes without required license
According to the official article, to include an Exchange Online mailbox in a retention policy created at Security & Compliance Center, the mailbox must be assigned an Exchange Online Plan 2 license. If a mailbox is assigned an Exchange Online Plan 1 license, you would have to assign it a separate Exchange Online Archiving license to include it in a retention policy. However, when create the retention policy, there is no such warning or notification to alert customers that the policy may not apply to certain mailboxes. It will cause customers' data lost because they may expecting the policy will work for all mailboxes.