User blog comment:Willbachbakal/(Summoner Spell Concept) Streamlining Summoner Spells/@comment-24862418-20161019171651/@comment-1330314-20161020024245

This isn't exactly the place to answer this question, but yes and no: Lethality is the replacement to flat armor penetration, except unlike current flat armor pen, Lethality scales with the opponent's level. If you have 20 Lethality and your opponent's at level 10, you get 14 flat armor pen against them, and if they're at level 18, you get the full 20.