So there are 2 different things. Execute, and kill indicators.
Regarding executes, Riven or Veigar are not truly executes, they don't deal a %Missing Health. Their damage is multiplied depending on the enemy missing health, and is capped if the enemy is below 33% (or is it 25%? Not sure, but just check the tooltip). So you don't need an indicator to know if you can execute the enemy, you know that if their HP bar is only 1/3rd (or 1/4th) filled, you will deal the maximum amount of damage possible.
Darius isn't an execute either, it's a flat amount of True damage, it's not more powerful or anything on low life enemy. He is just supposed to use it as a finishing blow because of the reset mechanism.
Same for Cho'Gath. Not an execute, just a flat amount of True damage.
Zed isn't an execute either, it's a delayed damaged amp.
But onto kill indicators. It's a case by case problem, and you need to ask yourself what it would do for a certain champion play pattern. An indicator adds reliability to a champion. Does he need this reliability? And since nothing is free, will it make the champ OP or what should be nerfed to compensate?
For instance, Garen is supposed to win against the Villain. That's basically the whole point of his passive: there is a single person in the enemy team against which Garen should always win. In this play pattern, reliability makes sense. On the other hand, if Darius manages a proper execution, he gets access to a huge amount of power, so it makes sense to gate this power behind a somewhat unreliable execution. If he had an indicator, the bloodrage passive or the reset mechanism would need to be nerfed.
Zed has an indicator because it creates an interesting decision point: do I go out and take the risk of the kill being denied by a late shield/heal once I'm out of range to add any more damage? Or do I stay in to secure the kill and take the risk of being killed myself, because I'm exposed for a longer period of time?
As mentioned above, Riven and Veigar are not true executes (contrary to Garen), and they have no particular benefit in using this specific skill to get the kill (contrary to Darius). If the enemy is below 33% HP, you know you'll deal the max amount of damage, so you can use the skill as soon as you have reached the cap, and if they are still alive, you can still add a different skill after that, no big deal. They clearly don't need an indicator.
Cho'gath is imho the least logical of the lot, I can't really justify him having an execute, especially now that he can stack his passive on minions easily with half the CD refunded. But on the other hand, his indicator is very slow, the visual effect takes a long time to appear when the target is in kill range, so most of the time you just use your ult when you feel like it and don't rely on the indicator anyway (at least according to my modest experience on Cho).