Part of this is likely because we're all still early in the preseason, and it's not easy to learn all of the stuff needed to succeed in one go, so even against a Mundo/Swain/Vladimir/Soraka team comp people might still not think to build Executioner's. Another issue, and one that's not exclusive to ExCalling, is that it's an extremely reactive buy on a class that is typically very proactive: up until recently, marksmen have only been building items designed to make them stronger in a way that benefited their champion, which is why we had the Infinity Edge rush and identical build paths in every game. It's not easy for most players to put their core build and item power spikes on hold and divert from their item path just because the enemy sustain champion is becoming a problem, just as many mages felt/feel uncomfortable at the idea of rushing Seeker's Armguard/Zhonya's Hourglass just because that was the best choice to stop Zed from snowballing in lane. On top of that, Executioner's feels bad to pick right now because it's a niche choice that's competing with Lord Dominik's Regards, an item that will usually work to full power in pretty much any situation (including against enemy mages and the like, which is an issue).
I'm guessing that this will become less of an issue within the next few months, once people realize for themselves, or from the pros, that Executioner's is necessary against sustain-heavy champions and that it's worth rushing even early on in most situations, and the Dominik's/Mortal Reminder clash will hopefully be made a bit more level with changes to Giant Slayer, which needs to be a lot less effective against squishies (e.g. "I built Rod of Ages/Rylai's Crystal Scepter, this item thinks I'm a tank now") while similarly effective against properly tanky enemies, so the end result would be that Last Whisper should build into two similarly niche items and not one super-niche pick versus a good-against-everything damage boost. Personally, though, I really dislike Executioner's Calling and Grievous Wounds in general, and while it was worth bringing back the item just to satisfy the community who wanted to see it again, while also testing out how to make GW better, I don't think the mechanic is ever going to be that good or intuitive. Right now, it only needs to exist because champions like Mundo or Swain have really poorly controlled healing, and would snowball in every game without healing reduction, yet the problem's been made worse because of Grievous Wounds, as those champions need their sustain to be strong enough even when hard-countered 24/7 (i.e. when someone has Executioner's Calling or even just a Morellonomicon). Even when it's doing work, the effect's not really that easy to appreciate, and even now it's just power without gameplay (at least Morellonomicon requires you to bring an enemy down to a health threshold first), so I think the end goal should be to remove Grievous Wounds completely from the game and make strong self-sustain a mechanic with innate counterplay, not one that's OP and impossible to deal with properly until you get the item that shuts it down.
an info box displaying how much healing it stopped. Similar to how 
tells you how much damage it prevented.
against a 
. We killed him pretty early with (with help from my jungler) and he came back to lane with only 
) . Neither of us was even level 6 yet. It let him outduel me for a long time in conjunction with 
despite the lead I got from (repeated) ganks.
against Malzahar, and to an extent zed. That doesn't make them bad items, executioner's calling is fairly cheap and it still reduces all other selfheals on the team, including lifesteal.
and the like.
if you're forcing your opponents to build items that specifically counter you and not as much your teammates.
, if you can get an enemy damage dealer to buy a 
