As for 06/06/2020

Vsix install does not work, see

https://docs.microsoft.com/en-us/sql/ssms/install-extensions-in-sql-server-management-studio-ssms?view=sql-server-ver15

Publish workaround

Zip the extension folder in C:\Program Files (x86)\Microsoft SQL Server Management Studio 20\Common7\IDE\Extensions\SSMSPlus as SFX package