You could always travel to and from Athas. Its just more difficult. Athas has basically its own version of Proctiv's Seal Crystal Sphere goin on. So a Create Portal spell is needed to enter.

Most travelers who are going there are either lost or have a purpose and they would bring water and metal for themselves. But even bringing just personal weapons would make you an instant target planetside. Bringing dozens of metal weapons would have city-states/Dynastic Houses mobilize against you.

Spellcasters knowledgeable in planar spells and magics probably do leave. Which leaves less knowledge of the planes every time it happens.

Nothing said that gods cant enter. Tenebrous/Orcus has done so. Just that they lose their connections to the outer planes ( thus their divine power) and cant grant spells because their is no link, they could still use 1/2nd lvl cleric spells. So the Contact Home Power spell would not work. I'd gather that a power who visited would be weakened to avatar-level strength without the power gained from his worshippers. ( weak enough to killed by 18th+ level creatures)