If the MSR work you talked about is this
Yes, I believe that's the paper we referred to during the interview.
I like language innovation a lot but things that affect the type subsystem in such a fundamental way are very hard to incorporate into an existing language like C#. It seems impossible to this on "on the side" as immutability is quite sticky; meaning you those sort of constraints need to be propagated around in the entire system to be beneficial.
Also, applying this to the hundreds of thousands of APIs in the .NET FX might be prohibitively expensive...