devtrackers.gg
Chiming in because I don't think I was incredibly clear and succinct in the conclusion: True damage gains on %health damage against tanks because armor increases more sharply than health does. The absolute power levels of BotRK/Kaken slayer aren't incredibly important, only the relative gains. And in general, those gains were greater against tanks than they were against non-tanks.
Some champions, like Yasuo, tended to buy purely +Armor (Death's Dance, GA), and others like Darius, would buy purely +Health. Those will also change your item goals.