Microsoft started rolling out the first Redstone 5 build (Windows 10 Insiders Preview Build 17604) to the Windows Insiders, who have opted into Skip Ahead. As there's a limited space into Skip Ahead and that's already filled, no new registrations are now being accepted.
If you are a Windows Insider in the Fast Ring, you already have a fresh preview build 17101, which is already started rolling out.
Windows 10 Insiders Preview Build 17604 is the first build from the Redstone 5 development branch. This build does not contain any new features, but some fixes are part of this build. As there are some major code changes going to happen is this new branch, there are a bunch of known and unknown issues.
If you didn't yet opt for Skip Ahead, you won't receive this build now. New registrations are already closed as there's a very limited space.
