This is dependent on the current parent task T146606 since currently, the action is logged as the reason, e.g:
It's nice for people to be able to know why their avatar was removed and also as a track record for people with special rights to make sure they're not abusing it. A reason field would allow someone e.g an admin to say something like "The avatar was inappropriate" or privacy concerns. This should only be given to those with right-avatarremove privileges since it'd be dumb to have a reason field for when you want to remove your own avatar specifically.