Salthorae
Imperial Mountain Dew Taster
Early Spelljammer just sort of had Athas there as a place to visit
it is mentioned in the Complete Spacefarer's handbook as a place that you can't go. Then it says (basically for DM's that DID want to include it), that it is so far away that no ones' ever been, there are no routes mapped anywhere to it.
So it exists, but visiting it? Only if the DM really wants that to happen because it's too hard to integrate as a setting, is basically the official Spelljammer stance.
There was the occasional two way portal mentioned in various Planescape sources
There was also the Gith invasion to Athas that left the Gith there, as well as a subsequent Gith invasion in the Black Spine adventure. So, portals exist, but as the adventure shows, it's very hard to open one from the Athas side.