Since this old thread still comes up in first page Google results on the topic of writing OBS plugins in C# (at least for me) I thought it makes sense to point out here that with .NET 7.0 and NativeAOT it's now possible to create native libraries from C# that can be loaded as fully functional OBS plugins. I have released example code with some explanations
here, you will also find more plugins
here on my resources page or on GitHub that are written in C# and that you can use as a starting point, source code is available for all of them.