TIME STOP SPELL (9º)
Depends very much on how you run HP.
If you play high HP as life force leading to increased binding energy between elements of your flesh, or any other HP-as-reality trope, you're going to find that slitting someone's throat while they are asleep or Time Stopped or paralyzed isn't a simple matter of a single action declaration--that flesh don't cut (easily). You'll get a dagger strike in at advantage as an auto-crit, exactly as if the target were paralyzed, and then your Time Stop ends.
You are, of course, free to rewrite all the 5E rules related to HP and paralysis and attacks against unconscious targets and called shots against conscious targets. If you do so you will find that Time Stop is not the only spell which has just gone way up in value. You'll probably also find your game is turning into GURPS: all the fighters will start making called shots against eyeslits, all the time during combat, because you've made bypassing HP a thing. That's a potentially fun way to play 5E but it is also a major rewrite.
To be honest... Most of my gaming still occurs in a game loosely based on 3.5 and 1e with some 5e spice. Characters max at level 5, but continue to gain abilities and can continue to increase damage (and armor reduces damage.) At high levels most people are killed/crippled by just one or two decisive blows that make it through armor, and bypassing HP to kill helpless targets is codified by official house rules.
However, I run a lot of infrequent 5e games as well. I definitely don't play HP as reality. Nevertheless, like I said in the post you quoted: in combat, I don't do auto-kills on helpless targets. I can assume that they are distracted by the other foes, so I simply follow the rules and such attacks are auto-crits.
(I might allow it if one threw caution to the wind and exposed themselves to other enemy combatants. But a kill stroke on a nonhuman would also be much more dicey in general, so I don't see this potential adjudication as game breaking. )
But out of combat, I don't see much justification for such a position in my games (since I don't do HP as reality). I'd allow Paul's example since it happens unexpectedly during negotiations.
If he did it in combat, I'd allow him an auto crit attack... The target is primed for battle, so the instant the time stop ends his adrenaline-amped instincts kick in and he backpedals, reacts defensively, or whatever. Plus, 12-30 seconds to carefully position yourself in front of him *without touching him* will be harder if he is in a defensive posture.
As far as out of combat goes... In the stupid "can torches light fires" thread I talked about how the DMG guidelines about when to roll the dice vs simple declare success/failure apply here. It's the DM's job to determine not just when to roll the dice, but when to look to the rules at all.
You don't need to look to the rules to know that torches light fires. In my view, I also don't need to look at the rules to know what happens when someone creeps up on a sleeping human and slits their throat. I might use rules to *get* the guy in position. But after that? The result of a slit throat? This would be resolved prior to engaging the rules.