there is a conversation you can catch between Alleria and Khadgar where it's revealed that he uses the wheelchair because Modera made it for him and Khadgar is a shy little bitch in love with her
that might be an in-universe reason but it still doesn't justify this shitty addition by the devs in the first place
you also have to ask yourself why a powerful Archmage like Modera would believe Khadgar needed a wheelchair like that