S&box is still in progress and using C# but it is now using the Source 2 engine instead. You can actually get invoted to it on Steam and play/build games for it now too.

It previously used UE4 and C# integration was all working with that but, as soon as they found out that they could get access to Source 2, they took the opportunity. Makes sense because the team is significantly more experienced with the Source engine since most of them have previously worked on or with Garry's Mod.