Finally Windows Server 2016 is supported for Skype for Business Server 2015.

You can find information here

On TechNet you can see OS table updated.

 

How to get the update
To add support for Windows 2016 to Skype for Business Server 2015, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015 from Microsoft Update. To do this, follow these steps:

1. Connect the server to the internet, and then start Microsoft Update or WSUS.
2. Run Setup.exe from the RTM build or a setup CD.
3. When you are prompted, select the Connect to the internet to check for updates option, and then select Install.

Note The installation window shows the download progress.

5.  After core component are installed, the Deployment wizard is started. In the wizard window, select the Install or Update Skype for Business server system option.
6.  Follow the remaining installation steps in KB 3061064.

IMPORTANT NOTE:

Microsoft Update won’t offer future Skype for Business 2015 cumulative updates to Windows 2016-based servers after the May 2017 cumulative update.

On Windows 2016-based servers, cumulative updates for Skype for Business 2015 Server must be obtained from the Microsoft Download Center. To do this, follow these steps:

1. Download SkypeForBusinessInstaller.exe from the Download Center.
2. Follow the installation steps in KB 3061064.

 

Hope this information help you!

Regards.