I have done all the searching I can think of, and I don't see anything stating which version of C# is available for Script, Expression, or other nodes where C# syntax is used.
So I keep trying to use things like string interpolation or declaring out values inline as arguments, and they work fine where I first try them, in VS, but of course the nodes error-out once I copy-paste and try to run them in flowgear proper.
If someone in-the-know could answer here, that would work almost as well as if the actual Flowgear Help Center had that information.
Since this question was asked, Flowgear has updated the help articles (Script, Expression) to list the version of .NET Framework in use. As the current* node version uses .NET Framework v4.5.1, the associated version the node compiles to is C# 5.0.
*As Flowgear update their nodes, this information is subject to change, so as mentioned in the question, the help articles are a good first place to look.