Metal working is canon · 7:17pm Nov 29th, 2015
Rainbow Dash had a metal wing in one of the alternate timelines, and it worked, allowing her to fly just as well as her normal one, and didn't seem to hold her back at all. This means that though we haven't seen it, Equestria can make wings, and possibly other, body parts out of metal that work like prosthetics.
People who write about her being depressed over no longer being able to fly are probably annoyed, but now they can write about her getting used to a metal body part after first getting it attached.
Like that "automail" stuff from Fullmetal alchemist. . .