Support for Windows Server 2016 has been added in Defender 5.9.1.
From the Defender 5.9.1 Release Notes:
"New features in Defender 5.9:
Rebrand to One Identity This product was rebranded as One Identity Defender.
Support for Windows Server 2016 Added support for Windows Server 2016 to One Identity Defender
Support for Active Roles 7.1 Added support for Active Roles 7.1 to One Identity Defender
Support for Active Roles 7.2 Added support for Active Roles 7.2 to One Identity Defender"