On Build 3105 / OS X 10.11.3:
I’m developing a couple syntax definitions using the new .sublime-syntax format.
When I have add one in the Packages/User folder, it shows up in View >> Syntax. When I add another into Packages/User, the original one disappears from the menu, and the new one doesn’t show up at all.
As soon as I remove the second .sublime-syntax file from Packages/User, the original one appears in View >> Syntax.
Also, from this thread, it looks like Packages/User-stored definitions should show up in View > Syntax > User: Sublime-syntax files won't show in View->Syntax as an option
However, my first one showed up in the root View > Syntax menu.