Earth-15 was an alternate Earth created after the events of Infinite Crisis which appeared to be slightly ahead of New Earth in its timeline as junior heroes (Jason Todd, Donna Troy, Kyle Rayner, Zod) have taken over or replaced their senior mentors.
Earth-15 was recently destroyed by a fully adult Superman-Prime who tunneled through its core repeatedly, which broke the planet apart after Superman-Prime directly killed many of that Earth's heroes such as the Justice League members of the Earth-15 dimension.
Destruction of Earth-15 by the fully adult Superman-Prime in his murderous search for his "true" "home" dimension
It is assumed that the Green Lantern and Atom who were native to this universe survived as they were not shown being on Earth-15 at the time of its destruction. These people had ways of getting off the planet before it was destroyed and were not attacked by Superman-Prime. It is not known if they did or if any other people survived Superman-Prime's direct assault on Earth-15 before it was destroyed.
The Earth-15 dimension still exists and well as the other populated planets of this dimension.