For those who don't know, Microsoft has been distributing a number of their Windows 10 builds via the "fbl_impressive" development branch, a branch created solely for Insiders and allows Microsoft to control what features are sent to the public for testing. There are many development branches which all focus on different things, however none of them are accessible by Windows Insiders.
Microsoft announced yesterday that they were suspending Windows Update and Windows 10 Insider builds as they prepare for the Windows 10 RTM. The next build offered to Windows Insiders will be using production servers, which means the next build we receive could very well be the Windows 10 RTM, or at the very least an RTM candidate, depending on when Microsoft plans to push another build out.
Microsoft has silently switched all Windows Insiders away from the fbl_impressive branch, and moved them over to the TH1 branch. The TH1 branch is where Microsoft is compiling RTM candidates, which means the next build delivered to Insiders can be nothing but an RTM candidate or the RTM itself. You can check to see if Microsoft has moved you over to TH1 by opening Regedit and navigating to HKLM>SOFTWARE>Microsoft>WindowsSelfHost>Applicability
and double clicking "BranchName". If you see "TH1" you are ready for the next build.