Unity provides its C# implementation. GitHub - Unity-Technologies/UnityCsReference: Unity C# reference source code.
That repository refuses Pull-Requests but it is very useful for the end-user programmers.
IL2CPP build pipeline consists of il2cpp.exe and C++ libraries.
I suppose that some of the C++ libraries contain NDA codes.
il2cpp.exe which is built on .NET Framework seems not to contain NDA codes.
I’d really like to contribute by making a Pull-Request, but I think the feedback would be more appropriate if Unity-Technologies made the source code available.