Riot Games have released an article detailing drop rates from Hextech Chests in League of Legends. The article seems to be sort of an announcement for their new Masterwork Chests that will be released in an upcoming patch as well.
For those not acquainted with League of Legends, Hextech Chest is a fancy name for a loot box. The upcoming Masterwork Chest will be a loot box that is slightly more expensive with slightly better drop rates. You can get them through scoring S- or higher in a match, limited to one chest per week and you can hold up to four in your inventory.
Keys for those chests are earned through the Honour system, which you are awarded by other players. You usually get this for carrying games and being a nice person to them. Spoiler alert: it doesn't prevent toxicity when things go south in a game. Now players have complained in the past about the chests' drop rates being inconsistent and this article with drop rates might be Riot's response.
Riot Games have incorporated ''bad luck protection'' in Hextech Crafting, meaning you can't open three chests in a row without getting a skin shard. This was not true in my experience, but the chests I opened were days apart from each other so there may be an internal clock of sorts. This system may only prevent bad luck if you open them in a quick succession, or this simply wasn't the case before patch 8.4.
It's also worth noting that duplicate skin and champion shards can drop and there is no bad luck protection for them. While Hextech Crafting is somewhat different compared to, let's say Destiny 2's greedy abuse of the loot box system, you should make no mistake - they are both gambling. They are different shades of the same entity, much like poker and blackjack.
There is one caveat with League of Legends' loot box system - vast majority of the game's skins can be purchased through normal means, without having to gamble for them. You can read the details on Hextech Crafting on Riot's support site.