Returning player after 4+ years, multiple missing champions and skins

So, I started playing LoL again like 2 days ago and was going over the champions in my collection and I started noticing that a LOT of skins and champions I clearly remember having, are missing. For example, Galio, who I used to play hardcore MID/SUPP, had Gate Keeper skin (which I got when a friend gifted me a mystery box, I remember it as clearly as yesterday, we were having a skype call and I screamed from the hype when it popped up) and Enchanted Galio skin (which I bought myself), or Battle Regalia Poppy, which I bought when it was on sale, or Jarvan IV Commando skin, which I got form a mystery box as well. When searching this problem, it seems I am not the only one who has been hit by this.