I was under the impression that only the amount of damage that exceeds 10% is reduced by half.
So if Poppy has 2000 hp and she gets hit for 500 damage 300 of that damage exceeds 10% of her hit points. So she is hit for 200+300/2 = 350 damage
It could be this, but the literal string provided to me doesn't suggest that.
Any damage dealt to Poppy that exceeds 10% of her current health is reduced by 50%. This does not reduce damage from structures.
The problematic word here is 'exceeds'. Typically exceeds is interchangeable with 'greater than', so the statement would read:
"Any damage that is greater than ten percent of Poppy's current health is reduced by 50%."
To change this statement to have the meaning that you think is true, I would say it that it should say this:
"The excess of any damage that exceeds ten percent of Poppy's currently health is reduced by 50%."
So if this is what Riot intends, then they should change the passive's description.