"For things like Conqueror or IE though you could accomplish the same sorts of results using % pen"
quoted from meddlers gameplay thoughts blog.
https://boards.na.leagueoflegends.com/en/c/developer-corner/BlxHAay0-quick-gameplay-thoughts-may-16
except true damage bypasses % damage reduction abilities in champion kits. these abilities are often short windowed, and give the user the chance for skill expression. they were put into these kits with the intention to be strong short windowed defensive steroids.
i dont think its fair that these abilities now get partially ignored, just because you think true damage is easier to understand than armor pen. in both cases (true damage and resistance pen), building health to counter this type of damage makes sense. i dont know whats supposed to be unintuitive about it.
but resistance pen doesnt cause as much collateral damage to other (balanced) game mechanics.
also have % damage reduction abilities.