Major discontinued feature of the SQL Server 2016 feature is that exclusion of 32 bit of the SQL Server. So now on, you have only 64 bit version.
Only other discontinued feature is, as usual dropping of the 3rd before compatible level which is 90 or SQL Server 2005.
However, even in SQL Server 2014, 90 compatible level which is also an discontinued feature.
I don't understand the possibility of discontinuing a feature which is already discontinued. Typically Microsoft keeps three versions of backward compatibility. however, in SQL Server 2016 , 4 previous versions of backward compatibility is kept as shown below.