I've frozen the language to C# 6.0 when C# 7.0 features were rolling out. I don't want to upgrade the language (atleast not now).

GC... I still have nightmares about our current GC setup, I don't think we want to further complicate it.

The only actionable item I see here is the xbuild deprecation (i assume this will affect our Travis CI eventually) We'll have to play around w it to see how to set it up.

I agree on C# 6.0 and xbuild, not sure about GC. Together with C#6.0 and xbuild, also underlying compiler is updated from "mcs" to "csc" (roslyn).

Is it possible to add the C# 7.0 support back in, since competitive programming sites aren't using the csc build because it takes too much memory and the compiler is killed by the OS?

can you give example of your problem, this does not make sense to me. so you need to use alternative to csc (mcs?) and support C# 7.0 in a code-base that calls into pythonnet?