This seems to explain why 3D support has disappeared: Google enforces HTML5 use on YouTube for Firefox 33 and newer – gHacks Tech News
(The rest of this post has been rewritten from how it originally appeared.)
Google is now requiring browsers to use the HTML5 player (by default) instead of the Flash player. Only the Flash player supported 3D video viewing options.
The original 3D Flash player support was added by a Google software engineer working in his spare time but did become an official feature in 2011.
What we know at this point is
- HTML5 is now the default video player on all browsers on the Youtube web site
- The HTML5 video player does not support 3D video
- 3D videos cannot be easily watched by casual Youtube viewers on the Youtube web site unless they use work arounds
- The workaround is to replace the /watch?v= portion of the URL with /v/, which forces use of the Flash video player with its 3D options
- Youtube videos embedded on other web sites continue to use the Flash player with 3D options
- Per a note on a 3D group, Google is aware that 3D is no longer working but has higher priorities at the moment. We do not know when or if 3D support will come back and if it does, will be in the near future or the distant future.
- Google has never supported 3D in the Android Youtube mobile app and mobile views accounted for 30% of my channel views.
- Google Chrome has enforced HTML5 player usage for a long time and has not properly supported 3D video viewing.
- Apple stopped supporting Flash. Google does not support Flash in Android. The days of Flash players are numbered.