It was said a few years back, and I'm clearly paraphrasing:
" True damage is a crutch mechanic and should not be put into the game in large quantities..."
- Someone on the balance team years ago.
Or at least rework some of the true damage to be a new sort of damage, basically like adaptive damage, but the damage type is based off of what resistance type your opponent has less of. That way you're doing a good amount of damage still, but there's counter play. that's just my thoughts. I'll call it "Altered Damage" for the example:
So instead of:
Infinity Edge:
20% damage is dealt as True Damage
Something like:
Infinity Edge:
25% of your damage is dealt as Altered Damage
Altered Damage Allows increased damage to Tanks, by passing their highest resistance and going straight for their weakness, without making their purchases partially invalidated. The above percentages are just based off of arbitrary numbers. In reality it may actually have to be something like:
1.0% True Damage = 2.0% Altered Damage
20% True Damage = 40% Altered Damage
Since Altered Damage is giving an increase to 40% of your damage, that is still resisted, where as True Damage is non-resisted damage. It's just something to think about. I'm not a game dev, I also haven't run calculations.
Edit:
Altered Damage & True Damage can even be used simulatenously to benefit each-other, to create a minimum amount of damage dealt (true damage) as well as increasing the ceiling of potential damage (altered damage).
20% True .D becomes ** 10% True .D + 20% Altered .D**