Warning: The following contains SPOILERS for Young Justice season 4.
The first three episodes of the second half of Young Justice season 4 reveal that Superboy is alive, suggesting the young hero was sent to the Phantom Zone. While the episodes do not mention the famous extradimensional prison from the Superman comics by name, there are several clues that hint at it. This development has been anticipated by fans, however, given that the fourth season of Young Justice was cryptically subtitled Young Justice: Phantoms.
The first half of Young Justice season 4 opened with the recently engaged Superboy and Miss Martian traveling to Mars to hold a traditional Martian marriage ceremony with Miss Martian's family. Upon arrival, the two were recruited to investigate the mysterious death of the Martian king, stumbling across a plot to release a viral weapon that would kill all of the green, red and yellow Martians in the process. Superboy seemingly sacrificed himself to destroy the weapon in Young Justice season 4, episode 4, "Involuntary," as Miss Martian could not sense him telepathically afterward and there was no way to recover his body from the magma flow in which he submerged himself before a bomb attached to the virus dispenser exploded.
It was teased in the midseason finale of Young Justice season 4 that Superboy might be alive in another dimension, after the magician Zatanna had a vision had a vision of Superboy where she "saw Conner's spirit..." and felt he wasn't at rest. This was confirmed in Young Justice season 4, episode 14, "Nautical Twilight," after Superboy awoke in a strange foggy realm where the only solid ground to be found was floating crystal platforms. Superboy quickly determined that he was alive as he could hear his own heartbeat, but was unable to touch anything unless he focused upon that task. The episodes which followed found Conner Kent exploring the strange realm, while trying to protect an unconscious girl (the Legion of Super Heroes member Phantom Girl) who was the only other person he had discovered trapped with him.
Ignoring the ties to the season's subtitle, there is much to suggest that the Phantom Zone is the other worldly space to which Superboy was transported. The geography resembles many depictions of the Phantom Zone from the classic Superman comics, and the inhabitants of the Phantom Zone are cut off from telepathic contact with people on the material plane. However, people in the Phantom Zone are also unable to touch one another, yet Superboy was able to carry Phantom Girl, albeit with great effort.
It's possible that Phantom Girl's phasing powers allow her to be held despite the physical laws of the Phantom Zone. It's also possible Superboy's tactile telekinesis allows him to "touch" objects in the Phantom Zone with his mind, since psychic abilities can operate within the Phantom Zone itself. In any case, whether or not Superboy and Phantom Girl are in the Phantom Zone, it seems certain that their story will continue throughout the rest of Young Justice season 4.
New episodes of Young Justice release Thursdays on HBO Max.