Does Randuin's Omen work against Infinity Edge's true damage?

The item could use a buff on its passive

It may reduce the damage before the conversion is calculated, in which case it would work against it.

There was a riot post saying that they were leaning towards that but idk what the final decision was.

If the item reduces critical hits, then it effectively, but not directly, reduces the true damage as well. The value of the true damage is based on the value of critical damage. If the value of Critical Damage is lessened, then it should follow the true damage will be calculated as lesser.

But depending on the item coding, it may not do that right now.

RITO pls give us a straight answer