Well he was inevitably gonna be changed due to the whole unifying thematic element to bring his power in line with that of the Shadow Isles. I mean, Hecarim, Karthus, Kalista and Thresh all abide to the same kind of stuff, and it doesn't negatively impact that. If anything "Master of Metal" was actually a contradictory title from a lore standpoint because he was imprisoned in a metal shell. The master of metal is also a slave to it?
Morde's identity is super powerful, super evil undead dude in a suit of iron that enslaves souls. Losing the metal aspect isn't the biggest detriment and honestly it was to be expected considering how out of place among the Shadow Isle's roster of undead he has been since...well Thresh came around and said "guys, this is how we look" and Hecarim was like "this guy gets it!"