Microsoft's Gabe Aul noted recently that the Windows Insider Hub would no longer be a default app installed with newer Windows 10 Builds. And true to his word, with this release of Windows 10 Insider Preview Build 10158, that is in fact the case.
The Hub is still available, just has to be installed separately and manually. To install the Windows Insider Hub do this:
Go to Settings then System and then Apps & Features.
Tap or click the Manage Optional Features.
Tap or click Add a Feature.
Navigate the list, locate Insider Hub, and click install.
Once the Windows 10 upgrade officially releases on July 29 to the general public, and you decide to continue in the Windows Insider program, the Insider Hub will be included in preview Builds again.