You know already that you can integrate your Exchange On-Premises with Exchange Online in an Exchange Hybrid configuration, allowing a consistent user experience wherever the mailbox is hosted.
But there was still few things missing. Few of them are (or were) the retention policies / retention tags, OWA / Mobile /ActiveSync policies which needed to be re-created on Exchange Online and maintain in both place.
An updated Exchange Hybrid Configuration Wizard had been deployed and is now allowing you can also transfer (one time transfer anyway) your retention policies from On-Premises (from Exchange 2010 to Exchange 2016) to Online.
As first step, only missing policies on ExO will be transfer. You will still have to maintain in both place if you make any updates on these policies (which usually is not the so frequent).
The option to transfer the missing retention policies is available when running the Exchange Hybrid Configuration Wizard (https://configure.office.com/Scenario.aspx?sid=33&uid=4541433136) by enabling the Organization Configuration Transfer show at the bottom of the wizard. You can take advantage of this capability with both hybrid configuration options (minimal or full).
And the result is shown below; first screenshots are the existing retention policies on my Exchange On-Premises and Exchange Online, and then after running the update HCW, the missing retention policies in ExO have been created.