So I just played a game as a cho and the tahm was able to survive my ult when he activated his shield even tho he had the indicator on him and his health was low enough beforehand to die. I don't understand how this is possible because of the fact that Cho ulti is TRUE damage which shouldn't be negated by anything except invulnerability right?
True damage doesn't ignore shields. The icon just shows that his health is low enough, but it doesn't account for shielding. If you had eaten him before his shield it would have finished him, but activating the shield makes that absorb your damage and protect him.
Same reason I save my W as Diana when fighting Cho. Popping it at just the right moment can dump on his ability to eat you.
If shields didn't negate true damage they would be near useless against certain champs.
The only "true damage" champion abilities that "ignore" shields are coded as executes like pyke and urgot r (damage numbers are for show, they just kill the champ unless that champ simply cant be killed or is spell immune)
Fountain turret does "Pure" damage which is a unique form of true damage that penetrates shields and invuln/untargetable while also preventing 
from procing
This is why I've argued against including execution indicators on abilities that aren't actually executions.
And here we are.
But also https://leagueoflegends.fandom.com/wiki/True_damage would be good to review.
As others have said, cho's ultimate is true damage, which doesn't ignore shields. Pyke's R ignores shields because it is something like "If the enemy is bellow X hp they get executed" so shields don't stop it because the enemy isn't really above the HP threshold to execute.
thank you all for your insight. I had the wrong idea as to what true damage actually was. My apologies for not realizing this.
