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In-world background: As part of its army, Country Bad Guys has a contingent of human slave cavalry. Country Bad Guys wants these humans to have an armor that will provide reasonable protection, but also not be top-of-line in case these humans revolted. They decide that they will give these slaves armor of human leather made from disobedient slaves as a reminder of Country Bad Guys' power. (Whether or not that idea is effective is beyond the scope of this question.)

Real-world background: I know that leather products have been made from human skin. I've also learned that historical leather armor made from cattle came from a specific part of the cow, the back where the hide is thicker. This latter piece of knowledge is the main reason I ask if human-leather armor is practical as I don't know if there'd be such a practical spot on people.

Question: Using Medievalesque technology, would it be possible to make useful armor out of human skin? And would it be practical?

For useful, I define that as providing at least around the protection-level of gambeson and or historical leather armor.
For practical I define that as not taking more than 10 people to make it from. (Country Bad Guys can't be killing all of its slaves.)

Some ideas I've thought about that may or may not be good starting points in answering the question:

1: Scale/lamellar leather armor allows for smaller pieces of leather to come together in a suit of armor, possibly decreasing the negatives of not having a ton of skin per individual to work with.

2: I thought about Ancient Greek armor and how they glued layers of linen together to create linothorax. I have to wonder if this might apply as a potential method.

Mentioned earlier, the skin on the bottom of [human] feet and on the palms of your hands is the thickest, which is on average 1.5mm thick. The thinnest skin on the body is found on the eyelids, which is on average 0.05mm thick. Male skin is generally thicker in all areas compared to female skin. (Source, note that I'm finding a variety of sources that claim as much as 4mm thickness, but this was the only one that seemed credible.)

Real leather armor is made from hides that are 4.8 mm (12 oz.) or thicker. (Source, This source does not cite where they found this info. It may only be an opinion.)

Summary:

What thickness of leather you use for armor depends on the piece of armor you're wearing. The piece on your torso will likely be heavier than the pieces on your legs (which need to be more flexible) or the pieces on your wrists (which need to be very flexible). Obviously, you want to use the thickest leather wherever you can.

Based on the above sources, leather armor is at least 4.8mm thick while the thickest human leather (from the not-too-abundant soles of the feet) is on average only 1.5mm thick.

Even if we assume human soles had a 4mm leather thickness (making them comparable to cow leather for armor), it's just the soles of our feet. That's a lot of feet, and a lot of sewing, and therefore a lot of weak points. Everywhere you have a stitch, it's a weakness.

Conclusion: Assuming all other things are equal (they probably aren't, as I recall human leather is more brittle than cow leather), it would either be a very light armor or a very weak armor due to all the required sewing of small pieces. Yes, medieval tech would allow its use for armor. However, a thick woolen cloth would probably be a superior armor to human skin.

As an alternative answer, have you considered making things other than armour out of human components? As stated previously, i don’t think human-leather armour could be very effective for the reasons i mentioned (it’d also be harder to repair damage if compared to gambeson as that can be resewn and be practically as strong as it was before the damage).

If your purpose is to remind the slaves of their oppressor’s power, i propose several ideas:

One is having tents or tarps made out of human-leather, this way the leather does not need to be armour-thick and might have more of a psychological effect as, unlike armour which can be taken off, you must either sleep in that tent or in the rain and mud, and you spend a lot more time sleeping than wearing armour.

Further options are cloaks, pillows and blankets made out of human hair for similar reasons as above. Either you use these or you sleep or walk in the rain and mud. Going back to the leather, you could make shoes out of human-leather, forcing the slaves to wear these or march bare footed.

You could possibly make eating utensils out of bone, not sure how they would hold up though.

There are likely more options i have not thought about but, as i say, if your purpose is to demoralise slaves using human components, i think this may be a better way of doing it than trying to make human-leather armour.

I would say that human-leather armour could never be as practical as gambeson. The reason being is firstly, the process of having to kill all these humans, skin them, tanning their hides and then boiling the leather to make it stronger requires far more effort than it is worth. Historically, we beleive leather armour was not really that common as we don’t have many historical finds of it. Now, abscence of evidence is not evidence of abscence (as leather may have broken down over time as it is organic) but it does provide an indication that they did not exist on-mass.

However, just because we can’t plausibly make leather armour out of human skin, does not mean we can’t make other types of armour.

One option is brigandine (or the early development, a coat of plates). Brigandine is a series of small metal plates that overlap and are fixed onto a leather backing (or cloth, but i will only refer to leather for the purposes of the question). The leather was not meant to be the protective part of the armour, whilst it was worn with the leather facing outwards, that was somewhat for aesthetics and to reduce the chances of your opponent being able to slip a blade between the plates of your armour.

In terms of protectiveness, bridandine is fairly effective, one noteable problem though is you can’t have arms on it as otherwise the plates would restrict your movement. This means your armpits are exposed, a potentially fatal flaw as a blade there could easily sever an artery or make your arm unusable. The issue can be avoided by wearing gambeson underneath to protect your arms. This would qualify as being effective, but not too effective in case your slaves revolted, and using human-leather as part of your armour.