How to get the older last whisper effect in season 6

The only item anywhere near it right now is Black Cleaver. It only reduces 25% of max instead of 35% but, well thats why nobody buys %pen items much anymore.

The game kind of turned from going for Armor Pen on ADCs to going for Crits. Not saging that is better just my observation. Aside from getting a trundle support or completely focusing your build on penetration, there isn't much way to get there like before.