As Windows 10's July 29 release draws near, Microsoft is temporarily shuttering ISO downloads and cutting off fresh installs of the preview OS. Gabe Aul, general manager of the Operating Systems Group's Data and Fundamentals team, has announced that within 24 hours, no new builds of Windows 10 will be available and preview keys will no longer activate fresh installations. Additionally, whatever build your test system is currently running will show as up-to-date when checking for updates via Windows Update.
Aul says this cutoff is being made so that the company can stress-test its final Win10 distribution channels ahead of the public release of the new OS. New builds will now hit Windows Update when the Operating Systems Group wants users to stress-test the system. After that testing is complete at some point in the future, new builds will become available for download as ISOs.
As of this writing, ISOs for build 10162 are still available for download, but it may be best to wait if you don't already have a test system running a Windows 10 preview build. Nobody is running these builds on production hardware, right?