I feel he’s too young to have undergone willing modifications, and it’s not like he was in the military to have been exposed to battle like Torb or Mcree for example.
So, how did he lose it? If he was “cursed” being in an accident that robbed him of his arm could be a pretty traumatic event for someone that young…
Probably while noodling or something
Im curious about this too. The Lore was no specific on how he lost his arm 
Same way Cassidy lost his.
Too much alone time 
So, can we confirm Shion took his arm? It’s implied in the new comic, but not actually shown.
I’d say so, the line seemed deliberate
His VA said we’d learn how he lost his arm in season 3. It might be in another lore drop or just through interactions, either way, it was almost definitely her.
It’s heavily implied that Shion cut it in the last comic.
Season 3 is going to have some awesome Lore! Cant wait! 