Back in 2015, Microsoft halted the support and development of some of its photo apps, including Lumia photo apps and mobile clients for the Photosynth service, allowing users to create interactive panoramas and upload them to the web. By the time of that announcement, the service itself was still accessible via a web interface, but in late 2016 Microsoft decided to shut it down too. And as of February 7, 2017, Microsoft Photosynth is no longer available.
Until the final date, users of the said service were able to download their creations to prevent their permanent loss before Microsoft took them down from the servers hosting them. The reason for Photosynth's shutdown according to Microsoft is low usage of the service. Here's the official note on the service shutdown:
We regularly review our portfolio of products and evaluate customer use and feedback to adjust our areas of investment based on how our services are being used. We understand that innovative photo experiences are important and are committed to development efforts in the experiences that mean the most to our customers.
It's sad to see this service gone — Microsoft isn't offering any official alternatives to it and the panorama capturing functionality in the Windows Camera app is still different in many ways. I personally used Photosynth many times back in the good old Windows Phone 7 days and it was really fun.