Microsoft has announced on its Lumia Beta Apps website that the service will be closed on September 30th. The note on the page also points out that if you need to save or view your previous feedback submitted to the site, you will need to do so before that date rolls around.
The reason for the closure is apparently down to "changes in the phone business at Redmond," the team have stated that the beta apps most recently pushed through the website will continue to be available on the Windows Store moving forward.
In addition, feedback for these apps will now be directed to UserVoice, which is an extension of the feedback portal used for Windows related feature suggestions.
If you have been using the site at all, head over to the Lumia Beta Apps website to review and backup your feedback.
Source: Windows Central
