Some abilities do magic damage without obvious reasons, like Garen's R or Fiora's W. Is magic damage canon? Does it have anything to do with demacian steel? And why would you give them magic damage if it wasn't canon? Hoping for answer by a Rioter.
i think it depends. In my interpretation magic damage is anything thats not physical ie a swing of your sword. I draw this conculsion from champs like sej, rumble and teemo. Sej does magic damage through ice which i guess is latent magical abilities were as rumble and teemo have no magical ability but are both ap magic damage. Rumble uses a variety of weapons and teemo uses poisons.
for your example of garen theres many talks that demacian steel has to have magical properties for it to be anti magic.
As for fiora i believe that one is for balance reasons similar to tris. No they lore wise dont deal magic but the balance team wanted ratios that would open up off builds and not make the champ too powerful with there main build.
Different damage types are just gameplay mechanics, influencing a champion's item synergy and their opponent's counter building options. Don't overthink it when it comes to the lore.
Regarding Garen's R, I am 99% sure he can not do anything remotely like that in the lore. If his ultimate did physical damage in-game, then opponents would be able to build armour and largely nullify his damage, making Garen useless as a juggernaut. Magic damage means that the ultimate remains a powerful spell even against an armoured opponent.
Regarding Fiora's W, her sword is just a sword, Demacian Steel or not. If Fiora kills you, the specific cause of death will be blood loss, not magic. Demacians don't like using magic anyway. I'm not sure why the ability does magic damage in-game though; perhaps to remove the black cleaver synergy?
I wouldn't say that the damage type is always canon(I mean Katarinas daggers deal magic damage).
Though in Garens case,he is literally making a giant sword out of light to smite the opponent,so the magic damage is pretty fitting here.
