JSP Specification :
Note: The Servlet 2.4 specification deprecates SingleThreadModel, which is the most common mechanism for JSP containers to implement isThreadSafe. Page authors are advised against using isThreadSafe, as the generated Servlet may contain deprecated code
So the vendors are free to implement using SingleThreadModel or any other vendor specific way. So it contain deprecated code
only if the vendor chooses to implement using SingleThreadModel.
So though class SingleThreadModel is deprecated the attribute isThreadSafe is not deprecated.