RobotsMetaTag Class

The implementation in SharePoint 2010 has changes from the previous version. The control is most likely only included for backwards compatability. According to ILSpy the control is implemented like below, which means it doesn't render anything. Don't see any output from it if I check the rendered markup so nothing seems to be injected either.
 
I would therefore recommend remove it from your master pages.
 
namespace Microsoft.SharePoint.WebControls
{
 [SharePointPermission(SecurityAction.LinkDemand, ObjectModel = true), AspNetHostingPermission(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal), SharePointPermission(SecurityAction.InheritanceDemand, ObjectModel = true), AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
 public class RobotsMetaTag : SPControl
 {
  [AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal), AspNetHostingPermission(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal), SharePointPermission(SecurityAction.Demand, ObjectModel = true), SharePointPermission(SecurityAction.Assert, ObjectModel = true)]
  protected override void Render(HtmlTextWriter output)
  {
  }
 }
}

Description

The Microsoft.SharePoint.WebControls.RobotsMetaTag class inherits from the Microsoft.SharePoint.WebControls.SPControl class which serves as the base server control for SharePoint WebControls. 

RobotsMetaTag is a reasonably unsophisticated control. When the control is rendering it will construct a SPWeb object using the current HttpContext and test whether the SPWeb.ASPXPageIndexed property is false. The ASPXPageIndexed property is responsible for relaying whether SharePoint WebForms are automatically indexed. 

The Usage Scenario

The most ordinary use of the RobotsMetaTag class is for derivation since it is common that users might not find the outputted meta tag to specifications. By default, this value output is CONTENT="NOHTMLINDEX" .

In the below sample, we are creating a new class that inherits from Microsoft.SharePoint.WebControls.RobotsMetaTag, and then manipulating its output.

C# Code Example

public class NewRobotsMetaTag : RobotsMetaTag 
{ 
protected override void Render(HtmlTextWriter writer) 
{ 
SPWeb web = GetContextWeb(HttpContext.Current); 
if (Equals(web.ASPXPageIndexed, false)) 
{ 
writer.Write("<META NAME=\"ROBOTS\" CONTENT=\"ChangedContent\"/>"); 
} 
} 
}

Visual Basic .NET Code Example

Public Class NewRobotsMetaTag
Inherits RobotsMetaTag
Protected Overloads Overrides Sub Render(ByVal writer As HtmlTextWriter)
Dim web As SPWeb = GetContextWeb(HttpContext.Current)
If Equals(web.ASPXPageIndexed, False) Then
writer.Write("<META NAME=""ROBOTS"" CONTENT=""ChangedContent""/>")
End If
End Sub
End Class

Adam Buenz
SharePoint Foundation MVP - http://www.sharepointsecurity.com