If I were to do it...
Adds a tiny layer of complexity to health, but small compared to some options. And it doesn't wind up massively inflating your hit point totals. It also makes crits more deadly without making them implausibly monstrous.
I think instead of having Wound Points as a separate pool (and having to figure out how many WP monsters of different sizes should have, and having the GM need to track two pools per foe), the easiest way to have a 'modular wound system' would be to make wounds conditions that critical hits can cause instead of extra damage.
One benefit is that if you had a player who wanted to opt out of suffering wounds for safety tool reasons, you could just let them use the normal 'crits do double damage dice' system the core rules have, while everyone else uses 'crits cause wounds but no extra damage.'
Mechanics
There are 5 locations that can be wounded, and 4 degrees of severity.
Locations
When an attack rolls a natural 20, it causes a wound to a random location. To determine randomly, you roll 1d6:
1 - mobility
2 - primary attack
3 - secondary attack
4 - stamina
5 - sensory
6 - attacker chooses
Mobility wounds affect legs, wings, or whatever worms and stuff use to move. It knocks the creature prone, and for the duration of the wound they're slowed.
Primary attack affects whatever the most threatening attack mode of a creature is (typically the primary hand for a humanoid). The creature drops whatever its holding in that limb, and for the duration of the wound that attack does half damage (or has disadvantage if it doesn't deal damage [or the save to resist has advantage if it both deals no damage and has no attack roll]).
Secondary attacks are any other attack, or just a spare limb (typically the off hand for a humanoid).
Stamina is meant to represent bleeding or some sort of reeling blow. Note the damage dice the attack deals. At the start of the attacker's next turn, roll those dice and the wounded creature takes that damage. This just happens one time. Then, for the duration of the wound, the creature has its maximum hit points reduced by the amount of the wound.
Sensory wounds affect eyes (or other primary sensing organs) and make you count as blind for one round. For the duration of the wound, the creature treats everyone as having concealment.
Severity
There are four levels of severity. If the creature struck is at 1 HP or above after the attack deals damage, the default severity is moderate. If they are at 0 HP, the default severity is serious. The creature struck can make a Constitution saving throw (DC 10 + attacker's proficiency mod) to reduce the severity by one level.
Light Wounds last until the end of the encounter or receive 1 die of magical healing. Mostly they just spice things up a bit during a combat.
Moderate Wounds last until you take a short rest or receive 2 dice of magical healing at once.
Serious Wounds last until you take a long rest or receive 3 dice of magical healing at once.
Critical Wounds last forever but can be healed with the (now 4th level spell)
regenerate. These attacks don't happen randomly, only with special effects like vorpal swords or high-level spells.
Special Notes
If a creature is already at 0 HP, you can use an attack to intentionally inflict a wound.
Certain creatures might be immune to wounds (like ghosts). Others might
require wounds to defeat - like perhaps zombies only go down when you inflict a stamina crit. Heck, maybe zombies suffer a crit with
any attack that deals more than 10 damage. You could have fun with it.
There could be a Rip and Tear sub-variant where the default severity of wounds goes up one step if the attacker's proficiency bonus is higher than the defender's, and goes down in reverse. This lets groups that want really over the top delimbing to do it without the PCs themselves falling apart to hordes of weak minions.
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And that's all you need. It fits into a page, and allows for the narrative effect of long-term wounds without really having a death spiral, and without requiring tracking a second pool of numbers.