@Joscelin, you're right about Silverlight being phased out.
Although Windows Phone 7 apps were a version of Silverlight (not the most recent at the time, mind you) there was never any Silverlight support in Internet Explorer on Windows Phone, Windows 8 (Metro) / RT, XBOX 360, or XBOX ONE.
That means that on the vast majority of Microsoft's own platforms there is no way to view older synths via the Silverlight viewer or the Direct3D viewer.
To be fair, even if there is a port of the older synth viewers to WebGL, it would not be viewable in IE on Windows xp or Windows Vista or Windows Phone 7 or Windows Phone 8.0 or on XBOX 360 since none of them have IE 11 and therefore do not have WebGL support (although Windows xp and Vista users can at least install Firefox, Opera, or Chrome).
Also, if an open source photosynth viewer is made, it could be ported to native apps for any platform without WebGL support in Internet Explorer.