But this is just initial step and doesn't add any new language features.

From the link:

  The Mono runtime we use in our Editor and Players has not been upgraded.
  The compiler still targets the C# 4 language. This is approximately what our old compiler supported.
  The compiler still target the .Net 3.5 framework profile we previously supported.

They have started the upgrade - https://forum.unity3d.com/threads/upgraded-mono-net-in-edito...

.net 4.6 and runtime and gc upgrades are all in the pipeline. but that doesnt help anyone right now.