This is something many users point out as a defect, but which is not.
It is the fact that despite the version of SQL Server you are using, the column produced by SET SHOWPLAN_XML or SET STATISTICS XML containing the query plan, is named “Microsoft SQL Server 2005 XML Showplan”.
Well, one can argue that the column could have been named such that it avoided any reference to the version when this functionality was released. Something like “Microsoft SQL Server XML Showplan”, for example.
Well, that would have been nice, but it wasn’t the case, and it wouldn’t be fair to change it after every version or even once. Such change would break the many commercial and non-commercial software that has been developed along these years, that relies on that column being called just that: “Microsoft SQL Server 2005 XML Showplan”.
So, this is not considered a defect and don’t expect it to change in the future.