I was browsing and found a question asking about Night Hags. If one member of a Night Hag coven is in the Ethereal Plane, can the others use the Shared Spellcasting ability? The OP asks about the rules for seperating a hag onto the ethereal plane. My question is what about all the other planes? Standard 5e rules as written.
The Planes, per DMG 43:
The various planes of existence are realms of myth and mystery. They're not simply other worlds, but dimensions formed and governed by spiritual and elemental principles.
The Hag ability reads
Shared Spellcasting While all three members of a hag coven are within 30 feet of one another...
The planes themselves are literally other dimensions, requiring strong magic to reach or communicate with. It's likely nonsensical to attempt to tell how far apart they are; distance is literally meaningless between them. The Ethereal plane is an explicit exception to this.