Microsoft advises remaining Dynamics 365/CRM Online customers on v8.1: Prepare to update
Microsoft has put Dynamics 365/CRM Online customers running version 8.1 on notice that they must update by July 31, 2018. The announcement on the Dynamics 365 Customer Engagement Team blog is in line with the team's policy of keeping customers within two versions of the latest. Customers will be transitioned to version 9.0.2, which includes a range new features and capabilities.
Administrators of organizations running Dynamics 365 version 8.1 will need to schedule an update by May 18, 2018, with a date for the changes to take effect set before the July 31 deadline. Organizations that don't schedule will be notified of an automatic update by Microsoft during a normal maintenance window.