The thing is, while 5e doesn't have negative hitpoints in the sense of previous editions, it
does still track damage beyond "reduced to 0", for the sake of calculating instant death.
[...] For example, a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack, she is reduced to 0 hit points, but 12 damage remains. Because the remaining damage equals her hit point maximum, the cleric dies.
The wording here is a bit confusing. It does say that she "
takes 18 damage", but then once she's been reduced to 0 it says that "12 damage
remains". I don't believe there's an official definition anywhere clarifying whether damage "taken" is "amount of damage to be applied to hit points", or "hit points - (damage)".
There's also the matter of "Taking damage while you have 0 hit points", which results in death saves. The wording still states that you're "Taking damage", despite not having any hit points to damage anymore. I guess you're just taking "0" damage? Which is... Fine, but sounds weird. And again, "If the damage equals or exceeds your hit point maximum, you suffer instant death". This doesn't specifically say "Damage
taken", so it seems like it's just referring to the number in a vacuum.
Actually, hang on... There
is kind of a definition of damage taken, right on the page before. "When a creature takes damage, that damage is subtracted from its hit points". Which... Doesn't clear things up a great deal, as that could potentially be read as either sequential or simultaneous.
Additionally, there's... ugh...
A tweet from Crawford stating that (in the context of barbarians and rage)
"Taking 0 damage is the same as taking no damage."By that logic, if you're at 0 hit points,
even though you have no more hit points to damage, damage "taken" still retains its numerical value. Otherwise you could never trigger death saves from damage while downed.