I really disliked this the first time I saw it. I was like "WTF is this?"

I can only think the reason why they did this was to compete with python, as our scripting languages that do not require a main/Main entry point.

Personally, it just feels wrong for compiled-based languages.

I think they could improve on C# with their namespace. Rather, for example, a typical C# file looking like:-

  using EFG;
  namespace ABC
  {
    public class XYZ
    {
     
    }
  }

They could just :-

  using EFG;
  using namespace ABC;
  public class XYZ
  {
    
  }

Pretty much taken from the C++ way.. but it saves on space availability.

The second option is what C# defaults to since a few versions ago.

Correct -- was added since .net 6, I believe.

Namespace declarations have been a thing in C# for a while

Yep. Point is it should have been added since Framework 4.