Whatever Happened to the Man of Tomorrow? is the final storyline appearing in the main Superman comic books prior to the Man of Steel mini-series which restarted Superman's history. The story details Superman's last stand against Mr. Mxyzptlk and a number of other villains including Bizarro, Lex Luthor, Brainiac, and Metallo. The story featured several classic Silver Age elements such as the Pre-Crisis version of the Fortress of Solitude. However, this story contradicts the events depicted following Crisis on Infinite Earths that saw the creation of New Earth; Due to this, it is possible this story depicts what would have happened to the Superman of Earth-One if the Crisis never occurred.
"Man of Steel is the dividing point between the previous canon and the current one; most comic fans refer to the two different versions as 'pre-Crisis' and 'post-Crisis', per Crisis on Infinite Earths being the major dividing line across DC's universe as a whole. The pre-Crisis stories were drawn to a close in Alan Moore's 'Whatever Happened to the Man of Tomorrow?'" (from the Man of Steel Wiki)