pocket ult specificly doesn't proc items because of that
giving an item to last hit creeps that late in the game is absolutly redundant, rare will be the case (specially from what type of stats that item and other might give) where your normal hit would not already kill them and the extra proc will not be ever used. you already cream anything with a few base items. extremely redundant.
For heroes, i don't agree either and here is why: in a type of game where mechanical prowess is less intensive and certain type of characters and builds for tanks are a problem and an item could be a solution is understandable. games where aiming and juking is such a baseline skill required to perform and you can pull out a daring escape at no hp with clever movement and use of the map. then award that skill, dont kill that opportunity from what it seams on the other side, you dropped your load on someone from cc and then can't chase correctly or used movement skill to get in over using the map now he has an edge to escape.
Would we want to create a "why try?" because you got tagged by that effect so escaping is suddenly very very hard. A case in point, even if its at a much bigger %, shiv is a problem character because if he gets a line on you and you are low you die without an external for the most part.
Execute mechanics are very scary design wise for a healthy game, its like all your hp has a 5% (for your case) debuff scalling since 5% of it doesnt exist.
The more i write the more i see problems with it. Going back at what icecream.tie said. pocket ult will not proc but everyone gets very low it suddenly becomes a if pocket ults you you RUN TO BASE or the off person with that item just finds an easy lethal on you.
Edit: went and checked to fact check this: every item that has hp related damage is current, to very much help burn tanks and healing (they are both on anti heal options but even if they were healing makes the effect tick more) or cd based max hp. There no missing hp yet. While i find some cases in other games interesting i do see that League has been far to bloated with it and being a case if the item timings match assassins or characters with it will blow people up from normal skills doing enough damage that the missing hp scalling ability will easly find lethal. and even Smite has had some various problems with it and very much removed missing hp damage from many problem cases for that reason but still having FULL execute abilities that exist as a draft option for resilient types of picks.