I feel like they wouldn't do it because it seems like Riot enjoys some people being to show how long they've been playing League, they added the legacy watermark, banner type thing over skins that aren't available anymore such as Definitely Not Blitzcrank. Although that skin is available through mystery gifting, you won't get the water mark in the loading screen because you didnt buy the skin when it was released. However, if you bought it in 2011, they want people to see that. (Same as Pax Sivir and TF, who many people just threw out the code or just never played League, so very few people have the skin)