I'm not sure what you are talking about because Ornn always had that % dmg on Max Health which comes from his 2 main dmg abilities Brittle and Bellows Breath (W). Here is all the changes on Ornn since his release.
V8.2
Bellows Breath Bellows Breath
Removed: Being Unstoppable icon unstoppable during the march.
Call of the Forge God Call of the Forge God
Cooldown increased to 140 / 120 / 100 seconds from 130 / 100 / 70.
V8.1
Bellows Breath Bellows Breath
Bug Fix: Brittle proc no longer apply its effect if the attack that would apply it is blocked.
V7.24
Bellows Breath Bellows Breath
Target's maximum health ratio reduced to 10 / 12 / 14 / 16 / 18% of the target's maximum health from 12 / 14 / 16 / 18 / 20%.
Cooldown increased to 13 / 12 / 11 / 10 / 9 seconds from 12 / 11 / 10 / 9 / 8.
V7.22 November 12th Hotfix
Searing Charge Searing Charge
Charge base damage increased to 30 / 50 / 70 / 90 / 110 from 20 / 40 / 60 / 80 / 100.
Shockwave base damage increased to 50 / 90 / 130 / 170 / 210 from 40 / 80 / 120 / 160 / 200.
V7.22
Stats
Base attack damage increased to 68 from 59.72.
Base armor increased to 33 from 24.04.
V7.20
Bellows Breath Bellows Breath
Target's health ratio changed to 12 / 14 / 16 / 18 / 20% of target's maximum health from 15 / 17.5 / 20 / 22.5 / 25% of target's current health.
Searing Charge Searing Charge
Terrain collision range increased to 150 additional range from 100.
Call of the Forge God Call of the Forge God
Removed: Headbutting the giant ram having a 0.35-second cast time.
V7.19
General
Recommended items have been adjusted.
Stats
Base mana increased to 340 from 300.
Living Forge & Master Craftsman Living Forge
Mini-shop UI now prioritizes health regeneration over attack damage when choosing which items to display.
Volcanic Rupture Volcanic Rupture
Mana cost reduced to 55 at all ranks from 55 / 60 / 65 / 70 / 75.
Searing Charge Searing Charge
Mana cost reduced to 50 from 60.
Big points are : His W is no longer unstoppable, deals 2% less dmg and has 1 extra second cooldown (oh no please god no I can't farm anymore) and his ultimate is 30 seconds higher at lvl 16. Gosh he is SO uninteresting to play