TLDR: It was on sale on steam store in early 2010 got removed and you can't buy it anymore.

This guide is just to explain how I got League on Steam since almost everyday for the past 5 years someone adds me to ask me this.

League is played by millions of people everyday, but on steam, the average League players are between 2 and 5 people each day, which makes it the rarest game on steam.

In early 2010, after the game came out of beta, at beggining of the Season 1, League was on steam store for 20€ and you would get the Collectors Pack (20 champions, runes and the Goth Annie Skin) that is on League Store + some RP.

Just a few weeks later, maybe 1 or 2 months League was removed with Riot was making plans to make the game bigger just as it is today and didn't need to pay steam anymore the fee's for having it here.