This new feature is based on, as we already know, it is common to use variations of boilerplate code to validate if the method arguments are null, for example:

public static void M(string s)
{
    if (s is null)
    {
        throw new ArgumentNullException(nameof(s));
    }
    // Body of the method
}

And now we can abbreviate the intention to check null parameters with !! like:

public static void M(string s!!)
{
    // Body of the method
}

This could also be used in checking indexer parameters with get and set:

public string this[string key!!] { get { ... } set { ... } }