FOR .NET CORE ADD:

Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);

EDIT:

else if (!ignorable)
{
    int c = Int32.Parse(hex, System.Globalization.NumberStyles.HexNumber);
    //outList.Add(Char.ConvertFromUtf32(c));
    Encoding encoding = Encoding.GetEncoding("windows-1251");
    outList.Add(encoding.GetString([(byte)c]));
}