Just yesterday, Microsoft released Windows 10 Insider Preview build 20211 to the Dev channel with a couple of new features. It offers access to Linux files in WSL, and you can now search file extensions in the Default Apps page in Settings.
Today, those on build 20211 are going to get KB4581021, bringing the build number to 20211.1005. There isn't anything that's new, or at least anything that Microsoft is telling us. According to the blog post, this cumulative update is meant to test the servicing pipeline.
Of course, this is something that Microsoft does from time to time. A key part of testing is making sure that cumulative updates actually work, because those have to be delivered monthly throughout the lifetime of a feature update. If all we get are full builds in the Windows Insider Program, then servicing isn't being tested.
Right now, Microsoft is working on finalizing 20H2, which is currently in the Beta channel. Once that's generally available, what the Windows Insider team does with the Beta channel should provide some insight into what we're getting in the spring. We know that 21H1 is no longer going to be a major update, so it's either going to be an enablement package like 20H2 and 1909, or it's canceled entirely.
Update: Apparently, there are some changes in this update:
This is a bit misleading.
The update removes the Meet Now feature I tweeted earlier. Meet Now can no longer be enabled after this update is installed.
explorer diff https://t.co/TyJzYlkSkA https://t.co/a0fEfasLLH
— Rafael Rivera (@WithinRafael) September 11, 2020