Not speaking to Kled here, but as an overall comment on "% health damage on fighters."
In the past I have tended to use % health damage as less of a "I kill tanks" stat and more of "I don't one-shot sqiushies" stat.
That probably sounds weird, but hear me out here:
Say your champion, Talon, is only good at killing Ashe and nothing else. That guy better be really good at killing Ashe, else he's going to be useless. Given how safe Ashe tends to be (especially when she has a Janna on her team) it's likely Talon is going to need some mechanics that result in him one-shotting her before she can respond.
Talon is viable, but Ashe has a bad time.
Now imagine that Talon had mechanics in his kit that allowed him to have some degree of success vs. champions who aren't Ashe. In this case Talon can be less good at killing Ashe because he can contribute to the team in other ways. Wooo, Joe's success case is now less binary and Ashe doesn't get one-shot as much.
% health damage is one such mechanic. It can allow a character who might normally be very focused on murdering squishies to have success cases against tanks, which means we can make their overall effectiveness against said squishies lower and possibly less frustrating.
Of course there's tradeoffs here. % health damage isn't always a good idea or the right thing to do, given the champion we're putting it on. Gains in one area often come with costs in another. Tank Ekko and Fizz showed us the dark side of more target agnostic assassins for example.
Side note: Tanks can still respond to % health damage by buliding Armor/MR. That means a tank doing well is still going to outpace a %health char doing poorly (assuming the right tuning). This is why %health true damage has always been so contentious.