FYI Response.Write is removed from MVC. 
There are two Solutions for .Net Core MVC is by using Content Result or by writing your custom ActionResult by overriding ExecuteResult method as below.
1. Using Content Result
 public IActionResult Index()
 {
  return Content("Hello World !");
 }
or even Just return string method
public string Index()
{
  return "Hello World !";
}
2. Custom Action Result:
 public class ResponseWriteActionResult : ActionResult
 {
      public override void ExecuteResult(ActionContext context)
      {
          var str = Encoding.ASCII.GetBytes("Hello World !");
          context.HttpContext.Response.Body.Write(str, 0, str.Length);
      }
 }
Usage:
public IActionResult Index()
{
   return new ResponseWriteActionResult();
}
References:
1.
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