Download Dragon Magazine # 353.
There is a big retcon of the Manual of the Planes. Demiplanes can exist in deep ethereal like in the Astral void. It depends form the specific demiplane.
They also say something I find absurd: you can cast gate to reach ANY demiplane... what an idiocy.
In 2nd ed demiplanes were only in the deep ethereal because that is the plane of potential and of creation. Astral isn't.
When it comes to demiplanes, I never understood while they could only be formed in the ethereal in 2e and astral in 3e?
Wouldn't it make sense for demiplanes to be able to form in both, but to be made of different materials? Kind of like building your house from either cement or brick. Each one would work but produce different results.
Also, I feel that certain demiplanes fit more appropriately into one or the other, and lose flavor if they are all converted in one direction. The observatorium, for example, fits well in the astral but would seem out of place on the ethereal. On the other hand, the isle of black trees would make more sense sitting in the ethereal. Additionally, the mazes would benefit greatly if they were diverse enough to be spread across both planes.