Over the last couple months, we've talked multiple time about the value of flat penetration items, as well as the importance of Void Staff. While the latter half of that discussion is still quite true, the first part is in question at the moment.
The biggest advantage that was posed by picking up items like Haunting Guise and Sorcerer's Shoes early on in the game was the incredible damage spike it gave you in the early part of the mid-game. While a Haunting Guise, Lost Chapter, and Sorc's Shoes might have less AP than a completed Morellonomicon, it often outcompeted it in terms of damage output, depending on the champion. For champions like Syndra or Taliyah, both of whome bring incredible base damages to the table, it was an ideal combination of stats that gave those champions a ludicrously strong power spike without asking for all the much gold. Why? Because against low MR targets (Read: AD Carries) the combination of Sorcerer's Shoes, Haunting Guise, MPen runes, and MPen masteries meant that all damage was being dealt as true damage. That's a huge deal when you have the ability to hit someone with an ultimate that deals 600 base damage like Unleashed Power.
That's the past, however, and Patch 7.9 definitely did a number to MPen builds. The first struggle such builds face now is the fact that vulnerable mid laners can now opt into an early Spectre's Cowl without sacrificing late game effectiveness since the item now builds into Banshee's Veil. The MR from the cowl singlehandedly offsets the penetration that Haunting Guise and Sorc's offer, making the combination of items far less effective than it was before in lane. Without the ability to bully lane, it's harder to get the roams into the bot lane that you're really trying to set up for, which brings us to the next point.
The more relevant reason that this build has fallen out of favor is that you can no longer rely on hitting every target for true damage. Patch 7.9 introduced a huge change to the game that went unnoticed by many players. That change was that every champion in the game now has Magic Resistance scaling, which is a huge deal for champions that were trying to penetrate exactly 30 MR previously. Ranged Champions, such as ADCs, now all get .5 Mr/Level, when previously they had none. By the time most champions could plausibly hit 30 flat penetration, ADCS will already be around level 9, so that comes out to 4.5 more MR which is, as silly as this sounds, quite a bit more than 0. It was already difficult for non-Syndra champions to truly one-shot champions with pure penetration, but now it'll be all but impossible, which brings the value of the build into serious question.
It seems the verdict is that traditional mage builds should get the nod for now, as most top-tier Korean players have stopped using the penetration heavy builds in solo queue, even Crown, who started the trend in the first place. If you really, really love Luden's Echo -- which every Syndra I get in solo queue seems to -- this will probably come as great news, since it's now reclaimed it's throne as the best second item to complement a Morellonomicon, so count on seeing it even more now than you did in the past.