The VHD specification was made public in 2006. The new VHDX spec was made available in 2012. Unfortunately, all the Microsoft links to it are now dead. Did Microsoft change their mind about making this public? Or, is it just an honest mistake?
The answer is that the link was later provided by Microsoft in a different location. As of August 2017, it is at https://www.microsoft.com/en-us/download/confirmation.aspx?id=34750