I don't really think there's that much more damage than in previous seasons, and a lot of it is that builds, combos and comps have become more optimised.
That said, when you see a Sion or Ornn get absolutely melted by champions they used to facetank for 15 seconds in the past, it does make you wonder; are the numbers that much higher than before?
And, no not really; but it's easier than ever before to get an incredible amount of penetration on either side, to the point that a lot of champs will just build % penetration as well as flat pen almost coincidentally.
Lord Dominik sends his regards
ADCs don't seem to hurt anyone until they pick up LDR, an extraordinarily powerful item that not only shreds 35% of the target's armor, but adds extra damage based on their health too. Once they get that, they start destroying even the tankiest of top laners.
It's absurdly easy, especially as LDR isn't exactly inefficient against low armor targets. It's not as powerful, but it's not like you're particularly handicapped by it either. It combines so beautifully with the Serrated Dirk that ADCs love to build early - as the flat pen is calculated after the % armor pen.
This makes buying defensive items excruciatingly inefficient. Tanks are supposed to scale into monsters who require you to focus them to take them down, but instead, you buy one item and every ADC bar Jhin and Kalista suddenly becomes Vayne.
Why buy armor if 35% of it disappears immediately anyway? Why buy health when that is used against you?
This is truly the crux of the problem; the damage isn't that out of line. Or at least, it wouldn't be if one was able to itemise against it by building resistances and health that were even vaguely efficient.
I hate this item.
Let's not forget how incredibly easy it is to deal true damage with mages and assassins.
Melee AP users as well as a few weird ranged ones (like Vladimir or Lissandra) can opt into Rocketbelt, which when combined with Sorc Shoes can give over 40 flat pen at full build. Void Staff, with its patently absurd 45% magic penetration, on top of that means you're doing true damage to anything with around 100 MR or less.
Why even bother building Magic Resist when Void Staff almost halves it immediately? It's ludicrous, and if you're a squishy, you cannot possibly stop Akali or Vladimir from doing true damage to you in the mid game.
Lethality is so hilariously efficient that it's barely worth me mentioning - and Eclipse gives some % armor pen too, just in case your assassin wasn't able to kill something that had perfectly itemised to beat it.
The actual AP numbers have gone down in recent seasons, but the damage hasn't. And that is because penetration is too efficient, too easy to get and too universally good.
Don't reduce the damage; make it easier to resist it with defensive items. Don't let LDR counter both armor and health, for example; make Void Staff expensive or massively cut the AP; and for the love of god, nerf Lethality.