As I was fetching today some older projects, I noticed 2016R4.1 does not accept rb projects when they are dragged over the icon.
It is still possible to open an rb project from the file menu, but it is less convenient.
It might be an oversight if it still opens fine with the Open menu.
2016r4.1 will definitely open older RB/RS projects; I’m refactoring a project from RS 2008 right now. I suspect that it’s just an oversight in the filetypes assignment. I’d file a bug report.