Arguably, there's less need for arenas in C# in most scenarios than in Go because of easy object and array pooling out of box with ArrayPool<T>, ObjectPool<T> (Sdk.Web workload) and stackalloc/InlineArray and co. You can also just use malloc/free directly with NativeMemory.Alloc/Free instead.
https://github.com/Enichan/Arenas
For C#.
Been trying to get into more managed memory in C#, so this might be something good for that.