Lethality in Runes should be reverted back to Flat Armor Pen
Leathality as a stat on items is fine, but changing all flat armor pen on runes to lethality goes against the whole point of the runes.
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Runes, unless you don't need the certain stats the rune type is efficient in the early game (i.e. armor against a mage in lane so you take scaling health), are meant for the early game to about level 6. Red runes though are almost always flat as AD/AP/AS/Flat Magic Pen (and formerly flat armor pen as well) is always useful/needed in the early game.
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Flat Armor Pen runes were early game runes and not mid to late game runes like they are now. You used them to get ahead in lane, before you finished your first item. And many of the most effective users of those runes were champions with strong base damage and/or percent damage, i.e. physical tanks and bruisers like Poppy
, Lee Sin 
, and J4
. Most assassins, with their better scaling and lower base damage, need flat ad in the early game to succeed and not fall behind, not penetration or lethality. -
Hybrid Pen runes were useful, not on most assassins who specialize in a damage type (except for Akali and Shaco, but both who prefer flat ad or ap to pen), but for mages who want to harass with AA in lane and/or help last hitting and for hybrid damage champions, many of whom aren't assassins.
Basically the change from flat armor pen in runes to lethality will have little effect on assassins in their rune choices as they will opt for AD (as they did usually even before the change to lethality), but will instead impact hardest non-assassin champions.
