On April 1 SQL Server 2016 Release Candidate 2 was released along with the SQL Server Management Studio March 2016 preview refresh v13.0.13000.55. This new version of SSMS uses the Visual Studio 2015 Isolated shell and doesn’t allow any 3rd party extensions like Tabs Studio.
Update (May 10, 2016): Thanks to Karl Burtram from Microsoft, Tabs Studio is now in the SSMS 2016 package load inclusion list.
Tabs Studio Blog (organizing Visual Studio document tabs) 