As of March 20th, 2019, ODM now migrates Full Access, Send As and Send on Behalf rights. See the snipped below from the ODM Administrator Guide for additional details:
Mailbox delegation permissions (Full Access, Send As, and Send On Behalf) can be transferred only in case the matched delegate exists on target. In case matched delegate does not exists, mailbox delegation cannot be migrated and an error will be reported. In case the target mailbox delegation is not set, the mailbox delegation will be transferred as is. In case the mailbox delegation as been set in target before the migration starts, rules and exceptions specified in Migrating Delegate Access Permissions section of On Demand Migration for Email User Guide are applicable to set mailbox delegation.