Let's Talk About Twitch's Passive (and How Little Damage It Does)
TL;DR at the bottom #Introduction
A DoT that deals true damage; it's an understandable design philosophy that there shouldn't be too much true damage since there's no way to counter it.
However, other champions with DoTs deal considerably more damage, despite resistances being in the game, and some dealing true damage? "They deal attack/magic damage" could be argued, but that's why the ratio is attached to them, and they also get armor/magic penetration. There is no "true damage penetration". Despite that, they still have higher base damages while also having two scaling factors (per level scaling and stat ratio). I can't find any ability specific statistics, but I can say with confidence from experience that DoTs on champions like Darius, Cassiopeia, and more I'll list later deal more damage than Twitch's passive, some with good reason and some not. Cassiopeia and Darius are just the most guilty.
#Skill Level
It goes without saying that the skill level needed to successfully deal damage with an ability will play a part in determining whether it deals more or less damage by default, omitting other factors not relevant to the topic. Let's take a look at the application of each major DoT, starting with the subject matter. For all of these, we will be using Level 3 stat values and not considering ability casts unless an ability applies the DoT.
#Twitch
##Application and Other Considered Values Auto-attacks, 550 Range, squishy, six stacks. Damage: 1/2/3/4/5 true damage based on level, per stack, for six seconds.
##Math With Alacrity and the +10% Attack Speed in Offense, he will have .82 Attack Speed, meaning it will take him about seven seconds to reach six stacks. 20 damage is dealt by the passive between the first auto attack and the sixth stack, for a total of 56 damage assuming Twitch does not continue to reapply it. This is lessened if Venom Cask was cast, as it reduces the time between the first stack and the sixth.
##Risk
There is huge risk at level 3 in a Twitch spending the time to apply six stacks, rewarded with a large amount of damage on his Expunge Contaminate. However, the damage dealt by the DoT is not rewarding whatsoever and honestly hardly noticeable. Even a health potion heals for 10 health per second and prevents a lot of potential DoT kills compared to other counterparts.
#Darius
##Application and Other Considered Values Auto attacks, 175 range, healthy, five stacks. Damage: 13-30(+30% Bonus AD) physical damage based on level, per stack, for five seconds.
##Math With +18 Adaptive Force from Offense, and Flex and Doran's Blade, he will have 25.2 Bonus Attack Damage and .65 Attack Speed, meaning it will deal 16 damage per stack (after armor is calculated, with the exception of someone starting cloth armor), 126 damage total (after armor is calculated) at full stacks, with the exception of someone starting cloth armor), and take 4.725 seconds to apply full stacks if the first auto is cancelled with Crippling Strike. 22 damage is dealt by the passive between the first and fifth auto attack, for a total of 148 damage assuming Darius does not continue to reapply it.
##Risk Depending on the matchup, it is an extreme risk to apply all five stacks as a melee champion, rewarded with bonus Attack Damage. On top of the reward of successfully applying five stacks, the DoT damage does quite a noticeable chunk of damage, more than double the damage of Twitch's passive, despite resistances reducing the amount of damage taken.
#Cassiopeia
##Application and Other Considered Values Skillshot with delayed cast, 850 range, squishy, one stack. Damage: 75(+90% AP) magic damage over 3 seconds.
##Math With +18 Adaptive Force from Offense and Flex, and Doran's Ring, she will have 33 Ability Power, meaning it will deal 80 damage (after magic resist is calculated and with the exception of starting Magic Resist in Flex or Defense).
##Risk The risk of applying this DoT is significantly lower, though counterbalanced by its mana cost and cooldown. Despite having a good reason to good amount of damage, even with as much early resistances as possible it deals more damage than Twitch's true damage passive.
#Teemo
##Application and Other Considered Values Autoattack, 500 range, squishy, one stack. Damage: 24/48/72/96/120(+40% AP) magic damage over 4 seconds.
##Math With +10% Attack Speed from Offense +9 Adaptive Force from Flex, and Doran's Ring, he will have 24 Ability Power and an attack speed of .76, meaning it will deal 26 damage (after magic resist is calculated and with the exception of starting Magic Resist in Flex or Defense). With the assumption he auto-attacks six times, he will do a total of 65 damage.
##Risk Considering top is his most commonly played lane, there is seldom any risk to applying his poison over and over. Even so, Teemo is the least guilty on the list, but still was worth mentioning due to him dealing more damage under the same conditions despite resistances.
#Honorable Mentions
##Brand His damage does %health magic damage, but is considerably harder to apply. Due to the complications in applying his DoT, and his DoT not even coming close to being his primary source of damage, he isn't worth the comparison, though it is still notable that his DoT deals considerably more damage and is %health damage.
##Malzahar His DoT being his primary source of damage, it makes complete sense that it would deal significantly more damage than Twitch's passive and be the highest damage DoT on this list. However, it was worth mentioning him as another example as a champion with a DoT that isn't true damage and deals considerably more damage.
#Professional Play and Platinum+ Solo Queue Twitch did not see a single pick in competitive play this season until MSI. In comparison, Cassiopeia and Darius were both picked multiple times around the world. This is, obviously, for more reasons than just Twitch's passive, but an increase in Twitch's passive damage could help him see a minor boost in pick rate. His early game is weak for more than one reason, but having even one low-impact ability is the difference between being picked and ignored in professional play. Where very minute micro mechanics can change the course of a game, having four abilities instead of five like every other pick is one, of many, contributions to his weak early game.
It isn't just early game either; statistics show that the average true damage done by Twitch in a game is 2,412. This is, of course, mixed in with red buff's burn damage, meaning his passive is doing even less than 2,412 damage per game.
#Reasoning Twitch is a very niche champion right now. He is, without a doubt, very strong, but the current meta just isn't letting him get much play (more later). It's completely understandable that true damage will deal less damage than physical/magic damage, but the damage output on Twitch's passive is so low that it's hardly worth calculating in fights. A passive definitely shouldn't be a core part of a champion's gameplay, it should just be a slight boost, but this isn't even a boost. It's an afterthought and a nice "haha" moment the 1% of times Twitch's poison actually gets a kill. Anyone who carries a potion can ignore Twitch's poison until he's level 9, then it only outdamages potion healing by 1. While a champion like Darius deals more than double Twitch's poison damage despite resistances. This isn't the major problem with Twitch's low pickrate, it's the meta, but this is one of many contributions to Twitch's weak early game. By no means should it be a game changing 5 true damage per stack at level one, but buffing it by even 1 damage per stack would be a huge improvement to his early game. It wouldn't make his passive feel overbearing, but it would at least make his passive noticeable.
#The Meta Aforementioned, Twitch doesn't have a high pickrate right now for more than just his passive. The meta just doesn't favour him in more than niche situations. He, without a doubt, can pop off in the right hands in the right situations, but he's still sitting at a 2% pickrate, the 3rd lowest pickrate of ADCs (right above Kalista and Kog'maw). We are in a high damage early game meta, where Twitch thrives in a tank meta and in the late game. 55.9% of games end before the 30 minute mark, and Twitch sees a huge increase in winrate between 30-40 minutes (going from a 51% winrate before 30 minutes to a 54% winrate after 30 minutes). A lot of the current laners bully him and provide a lot more in the early game than he does. Buffing his passive can help increase his early game success while still not making him even close to dominating the meta.
#TL;DR Other champions with DoTs deal significantly more damage than Twitch with their DoTs. This is, obviously, for good reason, but his passive is so underwhelming that it's more of an afterthought than an interactive part of his kit that Twitch players care to play around. Most of the time it's used for spamming ctrl+4 when the enemy has 6 stacks at 10 hp. He is bullied out of the meta for his weak early game already, seeing one of the lowest pickrates of ADCs. By no means is he weak, he simply just doesn't excel in the early game like other champions do. Buffing his passive by just one damage will ensure he sees more play without making him an overbearing, as he will be able to more successfully trade and deal a smidgen more damage during a fight.