User blog comment:UltimateYJfan98/create your own episode/@comment-5154078-20120624043750/@comment-5154078-20120624211221

Obviously, he wasn't part of the Justice League in the future. That narrows it down to three likely possibilities.

1. Superboy could have stayed with the Team and never switched to a new identity in ten years (hence he would not be listed as a character 'who should be introduced').

2. Superboy could have died in the struggle between the villains sometime prior to the activation of the time traveling device.

3. Superboy could have retired from the hero life at some point.

...My choice would have been #1.