Randy the Wizard needs to retrieve a sacred object that has been shifted into a demiplane for safekeeping. Randy knows that there a a number of traps and guards on the demiplane so he plans to use the Plane Shift (or Gate) spell to get into the demiplane, then cast Etherealness to bypass everything until he finds the object.
Once in its presence, quickly in succession drop Etherealness, grab the object, then Plane Shift/Gate back home.
The problem is, will this work?
The description for Etherealness says:
This spell has no effect if you cast it while you are on the Ethereal Plane or a plane that doesn't border it, such as one of the Outer Planes.
The DMG has this to say about demiplanes (p. 68):
Demiplanes are extradimensional spaces that come into being by a variety of means and boast their own physical laws. Some are created by spells. Others exist naturally, as folds of reality pinched off from the rest of the multiverse. Theoretically, a plane shift spell can carry travelers to a demiplane, but the proper frequency required for the tuning fork would be extremely hard to acquire. The gate spell is more reliable, assuming the caster knows of the demiplane.
[...] When a demiplane is connected to the Material Plane or some other plane, entering it can be as simple as stepping through a portal or passing through a wall of mist.
It's possible that a demiplane can be connected to the Material Plane, which is in turn connected to the Ethereal Border. This would also suggest that they are not part of the Outer Planes.
So would being ethereal on a demiplane work all the time? Only when the door to the demiplane is connected? DM Fiat?