Exhaust, red smite, death's dance, hextech gunblade. They all refer to damage, but not true damage. Riot doesn't tell you this, but true damage is not actually 'damage.' True is a modifier for 'damage' yet it's never treated in the same ways that physical or magic damage are. If this continues, then it shouldn't be called 'damage' anymore. It's just confusing to do this.
True damage also ignores resistance and all forms of damage reduction. It's a damage that will do exactly how much it says it would without question.
You can either only put on shield and have it soak the damage; it would still do what it says it would but you put up a dummy to take it for as much as it could before reaching your HP. Otherwise, you would need invulnerability such as Kayle's or Taric's R that would block all damage including true damage. Outside of these instances, there is no way to reduce or block true damage in terms of its raw number (though you can generally reduce its effectiveness by stacking HP, that is until you see bullshit like %max HP true damage).
.......
True damage is always maintained as a very reliable source damage but either doesn't scale or scales in a subpar fashion. So this design decision is probably just keeping to that.
Did you know that hitting rift herald's eye on Katarina with 
+ 
+ 5 stacks of Ravenous Hunter will completely fill her healthbar?
I thought that was interesting :D
LoL is honestly just a really inconsistent game in terms of mechanics.
