Starting in patch 8.13, you can now take a look at all your owned summoner icons in the Collections tab in the client. In addition to this, you can even see the rarity levels of each icon and where the icon came from.
This feature has been asked for by players for a long time, as sometimes you get icons from events and you forget where or why you got it.
It seems that the tiers for icons are similar to those of skins. They range from Rare to Ultimate, with Epic and Mythic sitting in between. Rare icons go out as simple event freebies, Epic and Mythic are rarer icons that you might have had to complete special missions for, or were only available for a limited amount of time.
There are only a handful of Ultimate tiers, but they seem like they go to very exclusive icons, like the icon for helping out in the League of Legends beta.
Icons bought directly from the shop seem to not have a rarity tier if they are available year round.
This new feature is on live servers now, so go take a gander of all your rare (or not) icons.