I have experience playing the low levels. I can briefly summarise the impact as follows:

• It will make encounters much harder. With many characters dying in combat, and possibly a few total party kills as well.
• This can be demoralising, but some players might be up for it.
• But something perhaps easily overlooked is that it removes a wonderful suspense building mechanic from gameplay. With current rules, a character going down leads to a change in tactics to keep the party member alive. By removing death saves, this falls away. And the only decision is: Is it time to flee, or can we still win this?

Characters in our party are regularly taken down (which would mean death with the house rule) and require in-combat healing to bring them back into the fight. Failure to do so in some cases would probably have resulted in total party kills.

In your comment on Dale's answer you consider:

players were encouraged to retreat if the going got tough

The problem with this is that there's not necessarily enough time to make an escape. The fact is that even as it stands (without the house rule): "level appropriate" monsters have a decent chance of 1-shot killing most level 1 characters. (When I say decent chance, I'm not even talking about needing a critical hit.)

NOTE That a single CR 1 monster is considered level-appropriate for 4 level 1 characters.

Consider a Specter (CR 1): It does 3d6 damage with +4 to hit. This is only an average of 10.5 damage. But it's special ability states that if a CON saving throw is failed, the target's maximum hit-points are reduced by the same amount (and if reduced to zero, the character is dead).

Of course if you and your players are up for a much more difficult challenge, then then feel free to tweak accordingly.

However, changing the death-saves rule is not the only way to do so. Remember that removing death-saves also removes a tension creation mechanism of combat. So instead of removing death saves, consider the following ideas:

• Have your monsters fight more "intelligently". Let them make good tactical decisions, and you'll see difficulty ramp up without any extra work.
• Tweak difficulty of encounters by adding monsters, or using stronger monsters. This might require a little more planning on your part, because it can be a little to easy to overdo it.
• Reduce the character's opportunities for rest a little. This means they rarely recover their abilities between encounters. It has a similar effect to the second point but is possibly a little easier to manage.

As far as I know, there are no magic items that fit your criteria, since they are either meant to be beneficial, or they are cursed - in which case the curse is not controlled by an outside force, such as your government. (There might be an item that I'm unaware of, maybe from an earlier version)

With magic items out of the play, the only thing that comes to mind is an ability that Way of the Open Hand monks gain at level 17:

Quivering Palm
At 17th level, you gain the ability to set up lethal vibrations in someone’s body. When you hit a creature with an unarmed strike, you can spend 3 ki points to start these imperceptible vibrations, which last for a number of days equal to your monk level. The vibrations are harmless unless you use your action to end them. To do so, you and the target must be on the same plane of existence. When you use this action, the creature must make a Constitution saving throw. If it fails, it is reduced to 0 hit points. If it succeeds, it takes 10d10 necrotic damage.
You can have only one creature under the effect of this feature at a time. You can choose to end the vibrations harmlessly without using an action.

10d10 (which is the effect on a saving throw success!) will probably kill any 5th-level character, unless you roll really bad. Or you just use the average result, which is 10*5.5 = 55. A barbarian in the group might survive this, but you can just "roll" really good so he doesn't make it. It's not like the players would know the difference (provided you roll in secret), and the effect will supposedly only be for deterrence reasons anyways, unless you're actively going for a TPK.

You could of course transfer this ability to a magic item, which could be capable of affecting multiple creatures at a time - if you so desire. After all, you as the DM are free to use abilities and effects that are not available to player characters.

Another possible solution would be using the spell Glyph of Warding and storing a high-damaging spell inside. Usually, I wouldn't permit players to trigger this spell on something like acting against the creators wishes, since that's a very complex trigger. But, since you're the DM, you can rule differently for your own sake.

Note that this glyph could be dispelled, whereas the monk's ability cannot.

Another possibility would be Geas, although its RAW version can be removed with Remove Curse or Greater Restoration, which violates your criteria. You could rule, however, that it can't be removed with one of these spells (although that deviates from RAW, obviously, which violates a different criterium).

Although that violates your criteria: have you thought about blackmailing the characters? For example "if you betray us, your wife dies" or something like that? Might be worth a shot, provided you find something appropriate for each character. You could also set the requirement for your players to provide something like this, unless the suicide-squad-aspect is not clear from the beginning and should be a surprise.