On January 12, 2016 support for many older versions of the .NET Framework ends.
Since the support policy links the .NET support to the OS, then if you are running a supported version of Windows older .NET Frameworks are supported by association. One would imagine that the RTM Windows Server 2016 will not be supporting .NET 3.5 SP1. With the installer for SQL 2016 requiring .NET 3.5 SP1, you might ask yourself, “how can Microsoft be requiring a piece of software that is reaching end of life?”
In the only place SQL Server is mentioned on this page, it says that “newer versions” of SQL Server were based on .NET 4. When I installed SQL 2016 on Windows Server 2016 today it required me to add the .NET 3.5 SP1 feature before the installation would proceed. This makes little sense to me, so if anyone can clue me in I will gladly update the information here. With IT shops likely launching a campaign to remove anything earlier than .NET 4.5.x from their networks soon we will need an official response from Microsoft on this.