You step into the border regions of the Ethereal Plane, where it overlaps with your current plane. You remain in the Border Ethereal for the duration. During this time, you can move in any direction. If you move up or down, every foot of movement costs an extra foot. You can perceive the plane you left, which looks gray, and you can’t see anything there more than 60 feet away.
While on the Ethereal Plane, you can affect and be affected only by creatures, objects, and effects on that plane. Creatures that aren’t on the Ethereal Plane can’t perceive or interact with you unless a feature gives them the ability to do so.
When the spell ends, you return to the plane you left in the spot that corresponds to your space in the Border Ethereal. If you appear in an occupied space, you are shunted to the nearest unoccupied space and take Force damage equal to twice the number of feet you are moved.
This spell ends instantly 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.
Using a Higher-Level Spell Slot. You can target up to three willing creatures (including yourself) for each spell slot level above 7. The creatures must be within 10 feet of you when you cast the spell.
bazinga
PHB says this is Conjuration, not Transmutation.
One key difference between legacy and this new version is the following:
New Version: "You remain in the Border Ethereal for the duration."
Old version: "You remain in the Border Ethereal for the duration or until you use your action to dismiss the spell."
So can the caster dismiss the spell or how do you interpret this difference of the old and new version of this spell?
You can still dismiss the spell, and if anything it's easier than it was before. The general rules on spellcasting in the 2024 Player's Handbook say that any ongoing spell can be ended by the caster at any time without using an action. All spells that had language like this specifying how you could end them have had that language removed as it's now redundant.
Would the Twinned Spell metamagic option apply to Etherealness?
As written, it's ambiguous and I think you can make a solid case either way. Personally, if I were DMing, I would allow it, since it seems within the spirit of how the new Twinned Spell was intended to work.
Unfortunately, that's a no in either case. For 2014, you can't twin it because the spell only effects yourself. For 2024, you can't twin it because it doesn't have upcasting.
It does have upcasting, though. As listed in the spell description, you can target up to three willing creatures (including yourself) for each spell slot level above 7.
I apologize, I should have been more detailed in my explanation. It does have upcasting, but RAW it doesn't qualify for twinned spell. Twinned spell says it can be used with a spell "that can be cast with a higher-level spell slot to target an additional creature". The upcasting on this exclusively increases by multiple targets. As a DM I would likely allow it but it doesn't work purely by RAW.