Microsoft has been releasing regularly test builds of Windows Server to Insider testers. But until Aug. 26, Microsoft has not shared any information publicly about what has been in those recent test builds.
Yesterday, Microsoft announced a new "vNext" preview build of Windows Server, build number 20201. In addition to listing a slew of features included in this test build, Microsoft officials also said in the blog post about it that this is a new build of the next Windows Server Long-Term Servicing Channel (LTSC) release.
LTSC releases of Windows happen every few years. The most recent Windows 10 client and server releases designated as LTSC were Windows 10 Enterprise LTSC 2019 (released alongside Windows 101809) and Windows Server 2019.
Windows 10/Windows Server LTSC releases are long-term supported versions of Windows released every two to three years. These releases get security updates for 10 years and don't receive feature updates by design. Microsoft has advocated that customers use the LTSC releases sparingly -- for mission-critical applications only -- and not for everyday use as a way to avoid updates. (Some customers do use them to avoid updates, however.)
Update: I'm hearing that Microsoft has decided that the client and Server LTSC releases may not happen together next year, after all. Server LTSC 'Next' is on track for a 21H2 release, but the Windows 10 Enterprise LTSC 'Next' client won't happen at the same time.
Microsoft officials have said features being tested by Windows Insiders in the Dev Channel (the former Fast Ring) are not tied to a specific feature update anymore. I've reported previously that my sources have said there may not be a Windows 10 21H1 release; Microsoft may only release a 21H2 Windows 10 feature update (possibly alongside a Windows 10X release in the spring of 21H1).