I haven't decided yet whether I'll remove old mixer avatars: if I do, it won't be for at least a few weeks after Jan 1. (New mixer avatars definitely aren't allowed.)
Let's assume you will allow old mixer avatars just like you allowed old animated avatars. Then, mixer accounts have pm access, this means someone wearing old mixer avatars can get a nice deal via pm to keep it. Will this mean promotion of the mixer, since the deal is not done publicly and there's no way to show that the person is being paid for wearing the avatar.