Before this season, true damage was completely unaffected by any damage modifiers. This included amps like Press The Attack and Coup De Grace, and also any damage reduction like Warwick E, Master Yi W and obviously armour/magic resist. The logic being that true damage is exactly what it says it is and no more or less. In my opinion, this is what true damage should be like. (Ahri's E is an exception to this rule since it's a specific interaction contained within the champion and only affects her Q's true damage.)
Come this season, this is no longer true. New items with damage amplifiers (Prowler's, Abyssal Helm, Riftmaker) all increase true damage and have done so since they were introduced. If this was just a change in how true damage worked, it would be fine, but none of the previously mentioned modifiers were changed. This means that runes, among others, simply don't function the same way as items.
Compare the description of Coup De Grace with Prowler's Claw and you'll see them both say "Deal x% increased damage" yet one affects more damage types than the other. Prowler's Claw in particular is an interesting case, as its active can be used on minions and monsters. Yes, this means it can amplify smite damage on monsters.
These differences shouldn't exist when they appear to have the same effect. The descriptions should either be updated to include/exclude true damage, or the effects themselves should be standardised as before.
Personally, I think we should go back to how true damage worked before. True damage as a concept doesn't work once it starts being influenced by external modifiers. The whole point of true damage in the first place is that it ignores these modifiers.
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