The next version of Windows Server – Windows Server 2022 – is now available in preview for download for MSDN subscribers (https://my.visualstudio.com/) and the evaluation center (https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server).
If you have a Software Assurance, you will be entitled to upgrade your server to Windows Server 2022 when available.
If you want to know more, you can have a look at this blog post https://cloudblogs.microsoft.com/windowsserver/2021/03/02/announcing-windows-server-2022-now-in-preview/