The Lumia Beta Apps website is shutting down on September 30 and will no longer be available to testers. Citing changes in the phone business at Redmond as the reasons behind the closure, the team have stated that the beta apps most recently pushed through the website will continue to be available on the Windows Store.
It's worth noting that should you have future feedback on the apps, it's recommended that reviews be submitted through the store. Feedback forms on the site itself have been changed to read-only. Unfortunately, there's no word as to what this means for future apps that may (or may not) be previewed on the platform.
Head over to the Lumia Beta Apps website if you wish to review the feedback you've submitted.
Source: MSDN; thanks, Vicky M., for the tip!