This is great news! Just curious if anyone reading this has tried compiling projects using .NET 4.* features?
I was trying to find some official docs about C# 6.0 - are you sure about that? I just tried:
public float MovingSpeed { get; set; } = 4f;
and got this error:
Unity-5.6.0b6 Linux
If you’ve got an old project, seems like you have to explicitly set the correct API compatability level.
You’re right, but I still can’t find the way to change it( tried to follow the FAQ from your link)

Thread title is a bit silly; it’s not in the regular 5.6.0b5 build, it’s in the experimental one posted on the subforum. You have to install that version.
That sounds better. Unity tends to do features on branches first, and only merges them in if there are no issues.