Is it possible to declare private methods inside an interface in a programming language?

Starting from version 8.0, C# introduced the ability to declare private methods within interfaces. This allows hiding auxiliary logic used in default interface methods from external access.

Example:

public interface IExample
{
    void PublicMethod()
    {
        PrivateHelper();
    }

    private void PrivateHelper()
    {
        Console.WriteLine("Private method inside interface");
    }
}

Before C# 8.0, interfaces could only contain method declarations without implementation, and private methods inside interfaces were not possible.