Years ago I wrote an addin to do specific things I want in SSMS. I use it all the time and have gone through the pain of working out how to get it running again each time a new version of SQL server breaks the addin model.
I've now installed SSMS 2016 side by side with 2014, but despite a lot of searching I can find nothing about how to update addins for 2016 and I have not found a way to do it. My addin continues to work in 2014.
To be clear, my addin uses the common approach of a class that implements IDTExtensibility2 and IDTCommandTarget:
public partial class SSMSAddin : IDTExtensibility2, IDTCommandTarget
/// <summary>Implements the OnConnection method of the IDTExtensibility2 interface. Receives notification that the Add-in is being loaded.</summary>
/// <param term='application'>Root object of the host application.</param>
/// <param term='connectMode'>Describes how the Add-in is being loaded.</param>
/// <param term='addInInst'>Object representing this Add-in.</param>
/// <seealso class='IDTExtensibility2' />
public void OnConnection(object application, ext_ConnectMode connectMode, object addInInst, ref Array custom)
Indeed, .Addin are no longer supported. You need a VSIX project building into "C:\Program Files (x86)\Microsoft SQL Server\130\Tools\Binn\ManagementStudio\Extensions[Your extension name]". There is not much information on how to extend SSMS 2016 but:
Here is some documentation for VSIX projects: https://docs.microsoft.com/fr-fr/visualstudio/extensibility/index
That forum helped me a lot: https://www.sqlservercentral.com/Forums/1802009/Developing-Extensions-for-SSMS-2016
You can't use Connect.cs anymore. The link between SSMS and your code now works with commands and packages as explained in the second link I posted.
Hope that helps