They nerfed Demiplane so that you can't use it as a prison for high lvl targets.

I don't think using it as a prison was ever the intended usage anyway, but it's still a very handy spell.

Miss the days that when you cast it multiple times it grew bigger so that Wizards and such could have their own places away from enemies and such permanently! 

While a cool idea, couldn't you accomplish something similar with Mordenkainen's Magnificent Mansion? Admittedly I think anything you bring in gets shunted out but there's plenty of ways for wizards to do extraplanar home base shenanigans. Also, you can still create new demiplanes which apparently doesn't destroy the old ones so that's nice at least. If you have enough time you can probably find a way to connect the demiplanes like individual rooms.

All aboard the NERF TRAIN!!!!!

CHUGGA CHUGGA CHOO CHOO!!!!

You can still trap enemies inside the Demiplane. All you need to do is make sure that the target is unconscious and therefore cannot "opt" to be shunted out when the door disappears. 

Quote from hjl247 >>

Miss the days that when you cast it multiple times it grew bigger so that Wizards and such could have their own places away from enemies and such permanently! 

 in what edition of the game was this a thing?

Can you just have unlimited Demiplane rooms? 
It says "Each time you cast this spell, you can create a new demiplane or connect the shadowy door to a demiplane you created with a previous casting of this spell."
That would mean you can have all sorts of separate "demiplane" rooms, right?

Yes you can

What do you do if you cast it and then you remain behind when the spell ends and you want to get out? Does the door just open up where you entered it when you cast it again from within the demiplane? Because the door isn't technically anchored to one spot, but I don't see any rules governing this

Quote from TheDavey >>

What do you do if you cast it and then you remain behind when the spell ends and you want to get out? Does the door just open up where you entered it when you cast it again from within the demiplane? Because the door isn't technically anchored to one spot, but I don't see any rules governing this

 

If you stay behind when the spell ends, you would need another way to get out. Casting the spell again doesn't create a door to the place you originally came from; it creates a door to another demiplane.

So you'd pretty much have to planeshift

Quote from TheDavey >>

So you'd pretty much have to planeshift

That would work. There are two other options I can think of, though they both require some degree of pre-planning:

1. You've already cast Word of Recall somewhere outside the demiplane, and now you cast it again to go back there
2. Someone else who knows the demiplane you're in casts Demiplane again (capitalizing on the last sentence in the spell description) and lets you out

Gate, of course, is also an option, but it's a lot more expensive than Plane Shift.

theoretically you might be able to still use it as a prison if im reading it right if you consider the door to the demiplane a "portal" which it is in my opinion combining it with the spell forbiddance to prevent them from being shunted out as that would be "using" a portal.

"For the duration, creatures can’t teleport into the area or use portals, such as those created by the Gate spell, to enter the area."

I've always thought of Demiplane as a way to create a safe, cozy, and private dimension, but now that I realize that other people have been using it to imprison other creatures, I've worked out a way to achieve this same effect, albeit requiring the creature to fail one or maybe two saves. This could become especially handy with how Imprisonment was nerfed!

• If the target you're trying to lure can magically teleport, either cast Mordenkainen's Private Sanctum (...for 365 days to achieve a permanent effect) or have a Bard/Cleric cast Hallow or Forbiddance (... for 30 days to achieve a permanent effect) within your demiplane to prevent teleportation or interplanar travel. If you have a Cleric that has Divine Intervention available, you can skip to the fourth bullet point.
• Lure your target into your demiplane. If you have Mordenkainen's Private Sanctum, Hallow, or Forbiddance currently active, you will not be able to magically teleport your target into the demiplane with, say, Gate. If you cannot physically force your target to enter the shadowy Medium door to the demiplane (for example, they are too large), Polymorph, True Polymorph, or Enlarge/Reduce might assist you with this.
• If the target you're trying to lure cannot magically teleport OR there isn't a spell currently preventing the target's teleportation, use Gate or some similar spell to shunt them into your demiplane. If you have a Cleric with Divine Intervention available, you can cast Hallow through that feature as an Action. Do this ONLY if you aren't planning to use Banishment as a way to disable your target from exiting the demiplane.
• Once your target is within your demiplane, cast Banishment on them OR cause them to become temporarily unconscious (potentially through the use of Sleep or extreme violence). While they are currently banished or unconscious, exit the demiplane and choose to end your spell, permanently entrapping your target while they are unable to "opt"  into being shunted out of the demiplane. If you are concentrating on Banishment, choose to end your concentration before the end of its 1 minute duration.

If you've any suggestions or see any flaws in this plan, do tell me, I want to perfect this strategy to a science.