An absent feature from Exchange Online (Office 365) is email address policies, which are put to good use in on-premises Exchange deployments to automatically assign the desired email address format to mailbox users.
The lack of email address policies to date has presumably been due to challenges of implementing them safely in a multi-tenant environment such as Exchange Online. Without email address policies, customers need to run PowerShell scripts (like Add-SMTPaddresses.ps1) to update the email addresses for their cloud-only mailbox users. Customers running a hybrid configuration, or otherwise running directory synchronization, can still use email address policies to update email addresses for on-premises objects and let the changes synchronize to the cloud.
Email address policies were briefly visible in the Exchange Online admin center, but weren't actually functional.
It seems the brief visibility of email address policies was in relation to changes to enable multi-domain support for Office 365 Groups, which makes use of email address policies but is managed via PowerShell.