Is health. Before you shit on me, think about it.
Let's say fiora attacks an ad who has 1000 health, and deals 25% of their health as true damage. Obviously the ad has 750 health left.
And in that same game, fiora attacks a tank that has 5000 hp, leaving them with 3750 hp.
Who's going to last longer?
That being said, max health true damage is pretty stupid, but there is SOME semblance of counter play as a tank, the people who get hurt the most by it.
[{quoted}](name=Onion the Fried,realm=NA,application-id=3ErqAdtq,discussion-id=5LtJt4Eg,comment-id=,timestamp=2016-03-14T16:02:22.211+0000)
Who's going to last longer?
Neither, they are both dying in 4 hits.
OP is right, health is a soft counter because there is more health left over at the end.
I think what most people don't realize is that counter building does not necessarily mean counterplay. The two are linked of course, but it's not like you can't have one without the other.
It's the difference between Annie and Vayne; one relies on outbuilding you and the other relies on outplaying you.
EDIT: Rearranged for clarity. Annie is the one who tends to outbuild you, Vayne is the one who tends to outplay you.
The counterplay to max health true damage is shutting down the bitches with a focused burst
#The Bitches: 
To a certain extent you're right. However these champions don't just hit you once, they duel you. You're going to last more if you have a smaller health pool than a tank or someone who built tanky. A smaller to medium sized health pool is perfect. Take it from someone who plays a champion that does that kind of damage to themselves. 
Vayne is a bit more deadly if you're a tank because it's much easier for her to deal that true damage to you. Fiora has to work for it and position herself, but I don't believe she was intended to be a tank slayer like Vayne. If Vayne condemns you into a wall when you're alone as a tank, then prepare to get shredded up. They're also mobile, while tanks are typically slow.
DPS counters Tank, Tank counters Burst, and Burst counters DPS. However every so often, in the lower ELOs, tank stacking will occur because so many tanks are preforming very well in the meta. They are quasi assassins. They deal damage, but they still can absorb so much damage themselves as well as have powerful lock down abilities. 
You in turn need high damage to deal with DPS, so it's a bad idea to build health anyways because you need to be putting out a lot of damage. For people that exclaim, "But Fiora has Riposte!!!" It's very easy to make the vast majority of Fiora players waste that, and I don't really even play top lane anymore.
I believe AS reduction is the only thing that works a little bit, but again, we need something that effectively counters tankiness to avoid people from tank stacking. That's an incredibly frustrating and stale comp for both sides.
