Lack of M Pen items and searchability in the shop

With the change from Flat Pen to Lethality a lot of people have been talking about builds, but something that isn't being talked about is why Penetration as a stat is not searchable in the in game shop. Just like crit or armor or lifesteal, pen should be a category to filter items by.

And on that tangent, Magic Pen is something that is currently lacking in the game. Sure we have Sorc shoes and Void staff, but Liandry's Torment is in such a bad spot no one buys it, and as a mage when you fight a squishy assassain like zed or yasuo and they have upwards of 160 mr because they have merc scim, edge of night and a maw how are you supposed to deal with that now that abyssal just gives a flat damage buff and doesn't help you deal with those ad threats? Possibly a black cleaver like item but an ap version? A way to reduce mr over time by a scaling value so as not to encounter the same problem with flat pen builds of last patch.