Well, I'm putting this on an NPC for sure

AC 19

The ac is broken when you equip it. IT only shows 18 it should be 19.

Did you remember to attune to it, magical bonus on items that require attunement will only appear if you attune to it

While attuned, it does not give the bonus to unarmed strikes.

If someone is cursed by demon armor, do they suffer levels of exhaustion for not being able to get a long rest? If so, do they just die after a few days from exhaustion?

The rules say: 'When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die). If you have any levels of exhaustion, the rest doesn’t reduce your exhaustion level.'(Xanathar's Guide to Everything, Chapter 2) So the armour wouldn't cause them to accumulate exhaustion or kill them in and of itself, but if they became exhausted by other means (environment, lack of sleep, magic, etc.) they wouldn't be able to recover until the armour was removed (or by greater restoration I guess).

Wearing this thing would be one of the most horrifying things I can imagine.

We're talking about a 65 pound suit of interlocking metal plates. Imagine never being able to take that off: constantly feeling the weight of it on your body. Psychological concerns of stress and claustrophobia aside, your bigger problem is physical, because over time you'd develop the most hideous pressure sores and bruises imaginable where the plates contacted your body. You'd never being able to take a bath or eliminate bodily waste in any way that gives you cleanliness or dignity, not to mention never really being able to get any kind of good night's sleep. You'd also never be able to properly wash the armor while you had it on, so the inside of the armor would accumulate over the months with grease, body odor, oil, blood, hair and whatever bodily waste you now constantly have to smell, developing horrible rashes from contact with this filth. Gnats and other small insects would swarm around you, and when it got bad enough, they might even breed in the pockets of filth coating the armor. Did we even mention temperature? What about when it's blazing hot and, God forbid, you have to move in that thing? Sweat would be pouring down every part of your body, stinging your rashes and open sores, running into your eyes (you can't take the helmet off). I don't know if any of you have ever had swamp-butt before, but it's NOT pleasant. Let's not even talk about the horrible athlete's foot you're guaranteed to get so that every step would be hideously painful. Some poor guy who spent six months in that thing would literally want to die just so it could end.

Prestidigitation cleans you. so that helps. also it looks like there are gaps in the plates (as per the artwork) so you can use the bathroom in the armour so again not quite as bad. Id invest in some heat and cold resistance rings to ease the temperature discomfort.

Hooooolllllyyyyy crap this cool

What if the wearer dies? Could it be looted off their body without the use of a remove curse spell? What if the dead wearer has a clone spell active? Could the clone loot the armor off of the original body without a remove curse spell?

Quote from Diothycles >>

Prestidigitation cleans you. so that helps. also it looks like there are gaps in the plates (as per the artwork) so you can use the bathroom in the armour so again not quite as bad. Id invest in some heat and cold resistance rings to ease the temperature discomfort.

 I don't think there are gaps. (Evil laugh.)

what is the armor made of? and can it be crafted? does remove cruse remove the disadvantage against demons? 

Quote from akidthatwanttousethebuildersheet >>

what is the armor made of? and can it be crafted? does remove cruse remove the disadvantage against demons? 

 I belive remove curse only allows you to remove the armor. about ther other questions you could easely come up with something in your campaing.

¨I assure you demon, I have no pride!¨
Also, this actually would be good for a Paladin or Cleric with heavy armor proficiency. Maybe the odd Bard if they have it, too. 

the gaps appear to be covered with either cloth or pieces of chainmail, in their case, probably only enough to fit like maybe a pipe cleaner through, and it would take a lot of pipe cleaners.

there are rules for making magic items, so if your dm allows you to do so and you have the capability, you can. A lot of magic items, especially more powerful ones, take additional resources to craft, so you would need to figure that out. DMs, I would get creative with what is required like a piece of one of each of the demons' rivals, like a devil's claws for the gauntlets, dragon scales for the plates, and a celestial's skull for the helmet.

as per the soiling yourself debate, that is oddly fitting

I want to build a character with this

Does remove curse allow for the armor to be removed just that once or afterwards does it get stuck on them again and require another spell?