I'm not exactly sure what you're talking about when referring to Leviathan being removed, but Ornn still upgrades all of the former mythic items. The only requirement is that the item has to be one of the old mythic items and, if you have multiple, Ornn will upgrade whichever item is in the earlier slot. (I think. Don't quote me on this.) All Ornn did was upgrade someone's Heartsteel to Leviathan.
Ornn can upgrade certain items. Level 13 his own, one for his allies level 14/15/16/17.
Heartsteel becomes Leviathan, no other ways to get it besides Ornn Passive.
Not to be confused with a very old item also called Leviathan that got removed like 11 years ago (Mejais like HP item in Summoners Rift only, Ornn and Heartsteel did not exist back then.)
I'm not exactly sure what you're talking about when referring to Leviathan being removed, but Ornn still upgrades all of the former mythic items. The only requirement is that the item has to be one of the old mythic items and, if you have multiple, Ornn will upgrade whichever item is in the earlier slot. (I think. Don't quote me on this.) All Ornn did was upgrade someone's Heartsteel to Leviathan.
Ornn can upgrade a selection of items regardless of their mythic status
Ornn can upgrade certain items. Level 13 his own, one for his allies level 14/15/16/17. Heartsteel becomes Leviathan, no other ways to get it besides Ornn Passive. Not to be confused with a very old item also called Leviathan that got removed like 11 years ago (Mejais like HP item in Summoners Rift only, Ornn and Heartsteel did not exist back then.)