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.
There was also Mono UE [0] - which was being developed by some of the Xamarin folks.
I’ve not played around with any game development for a long while - does C# end up popular here just because people are familiar with Unity? I definitely get that C++ is not the most productive language - though improving perhaps.
Simply loading the .NET Core CLR is not difficult [1] - but unless things have changed since I last tried binding C++ interfaces to C# is time consuming, especially if you want to be able to implement an interface in managed code.
Part of C#'s popularity with indies is due to the legacy of XNA. Thousands, probably tens or even hundreds of thousands of indies got started with XNA and released games using it, so it's natural to stay on C# whether you move to FNA, MonoGame, Unity or Godot
Maybe ChatGPT6 can port the UE C++ codebase to C# and we can call it UE6