Permanent means it lasts until one of its two end conditions are met
The true polymorph spell says:
Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into a nonmagical object, or the object into a creature
Creature into creature: the slug
By choosing the creature, you have made the creature into the target of the spell. After its transformation, it remains the target of the spell.
The spell lasts for the duration [Concentration; up to one hour], or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the spell lasts until it is dispelled.
This spell has no effect on a shapechanger or a creature with 0 hit points.
There is some disagreement about whether concentrating on the spell for the entire duration means that the "or until the target drops to 0 hit points or dies" still applies. Fortunately that is largely irrelevant because of the next sentence; the spell has no effect on a creature with 0 hp. If the target of the spell drops to 0hp (which it will when you step on the slug), the spell ends. This is true even if the spell has been concentrated on for the full duration; it is an overarching condition that can end the spell. Thus while concentration still applies the spell has four end conditions; it will end when the concentration ends, when it is dispelled, when the target drops to 0hp, or when the target dies. After the spell has become 'permanent', it has only two end conditions - being dispelled or losing its effect if the target is at 0hp.
One might argue that "this spell has no effect on a shapechanger or a creature with 0 hit points" refers to targeting rather than duration, but this claim is not supported by context. First, if the clause was about targeting it should go in the first sentence of the spell description, which is about targeting, before the effects of the spell are described ("you transform"). If it was about targeting, the clause would not go in the second paragraph, after the section on duration. Second, it does not say "This spell cannot target a shapechanger or a creature with 0 hit points". It does not say "You cannot choose..." It does not say "This spell will not affect..." (future tense). Rather, it says (present tense) this spell has no effect. Thus anything that reduces the original target creature to 0hp will end the spell effect.
That the effects of the spell end when its target is at 0hp is RAW. Unofficially, it is RAI as supported by a JC tweet1.
Creature into Creature...
The target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn’t reduce the creature’s normal form to 0 hit points, it isn’t knocked unconscious...
So, you wait out the 61 minutes for your foe to be permanently a slug. Then you step on it, reducing the slug to 0hp. That triggers one of the two remaining end conditions of the spell; it has no effect on creatures at 0hp. The former slug returns to its normal form under the 'creature to creature' rules, with the hp it had before it transformed, less any damage that carried over from you squishing it as a slug. You now have your original foe, down a few hp. Perhaps you should have kept the slug, letting it live out its days in a terrarium, and had it die of old age rather than damage? Or maybe we should have made the foe into an object?
Creature into object: the cup
Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into a nonmagical object, or the object into a creature
The target of the spell is the original creature. The object it transforms into is also under the effect of the spell. The original creature is still the target, even though it exists at the moment only in a conceptual, not a physical sense. You know that the game treats the creature as if it still exists because it will be restored when you use dispel magic, and because the creature itself can still be seen with truesight.
If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form, as long as the object’s size is no larger than the creature’s size. The creature’s statistics become those of the object...
Since the creature's statistics are now those of the object, the creature's hp are those of the object as well. This is important because, as above, if the object's hp become zero, then the target creature's hp become zero, and this will end the spell.
Normally the game does not worry about the hp of an object unless something is damaging it. The DMG section on statistics for objects (246, 247) focusses entirely on how to break or destroy them, including saying:
Hit Points. An object's hit points measure how much damage it can take before losing its structural integrity.
Damaging the object should at some point reduce the target creature to 0hp and thus end the spell effect. If the cup is stomped on, it will at some point 'lose its structural integrity' and reach 0hp. When the spell ends, the creature is restored.
If the creature had the statistics of the object, and was restored when the object reached 0hp, does that mean that the original creature is at 0hp when they are restored and you have killed your foe (or at least forced them to start making death saves)? I don't think so, based on a comparative reading of the 'creature into creature' and 'creature into object' sections of the spell (emphases mine except for the heading).
Creature into Object. If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form, as long as the object’s size is no larger than the creature’s size. The creature’s statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.
Note that the 'creature into object' description does not specify what hp the creature is at when it is restored, but only says it 'returns to its normal form'. Since the 'creature into creature' section occurs first, I believe that it is defining what 'returns to its normal form' means: 'the creature returns to the number of hit points it had before it transformed'. The 'creature into creature' section then further elaborates that the original creature takes the excess damage that went beyond what was needed to reduce the new creature to 0hp. The 'creature into object' section does not specify such a thing, so the excess damage rule appears to apply only to creatures.
Thus breaking the cup restores the original creature, which returns at the number of hp it had at the time the spell was cast and having not even taken excess damage from the destruction of the object.
Further evidence that the destruction of the object is not meant to kill the original creature is the phrase, "the creature has no memory of time spent in this form", which would be unlikely to be necessary if it was dead.
This is a 9th level spell, and a powerful way to manage enemies that fail their save. It is not, however, a one fail = death solution to all foes.
1RAI tweet:
@DMJazzyHands once a true polymorph spell is made permanent from concentrating for the full duration, does reducing that creature to 0hp still cause them to revert to their original form?
@JeremyECrawford The text of the spell says it has no effect on a creature with 0 hit points. That statement is made after the bit about lasting until dispelled. At 0 hit points? The transformation ends.
