challenges.json

[
  "12AssistStreakCount",
  "abilityUses",
  "acesBefore15Minutes",
  "alliedJungleMonsterKills",
  "baronBuffGoldAdvantageOverThreshold",
  "baronTakedowns",
  "blastConeOppositeOpponentCount",
  "bountyGold",
  "buffsStolen",
  "completeSupportQuestInTime",
  "controlWardsPlaced",
  "controlWardTimeCoverageInRiverOrEnemyHalf",
  "damagePerMinute",
  "damageTakenOnTeamPercentage",
  "dancedWithRiftHerald",
  "deathsByEnemyChamps",
  "dodgeSkillShotsSmallWindow",
  "doubleAces",
  "dragonTakedowns",
  "earliestBaron",
  "earliestDragonTakedown",
  "earliestElderDragon",
  "earlyLaningPhaseGoldExpAdvantage",
  "effectiveHealAndShielding",
  "elderDragonKillsWithOpposingSoul",
  "elderDragonMultikills",
  "enemyChampionImmobilizations",
  "enemyJungleMonsterKills",
  "epicMonsterKillsNearEnemyJungler",
  "epicMonsterKillsWithin30SecondsOfSpawn",
  "epicMonsterSteals",
  "epicMonsterStolenWithoutSmite",
  "fasterSupportQuestCompletion",
  "fastestLegendary",
  "firstTurretKilledTime",
  "flawlessAces",
  "fullTeamTakedown",
  "gameLength",
  "getTakedownsInAllLanesEarlyJungleAsLaner",
  "goldPerMinute",
  "hadAfkTeammate",
  "hadOpenNexus",
  "highestChampionDamage",
  "highestCrowdControlScore",
  "highestWardKills",
  "immobilizeAndKillWithAlly",
  "initialBuffCount",
  "initialCrabCount",
  "jungleCsBefore10Minutes",
  "junglerKillsEarlyJungle",
  "junglerTakedownsNearDamagedEpicMonster",
  "kda",
  "killAfterHiddenWithAlly",
  "killedChampTookFullTeamDamageSurvived",
  "killParticipation",
  "killsNearEnemyTurret",
  "killsOnLanersEarlyJungleAsJungler",
  "killsOnOtherLanesEarlyJungleAsLaner",
  "killsOnRecentlyHealedByAramPack",
  "killsUnderOwnTurret",
  "killsWithHelpFromEpicMonster",
  "knockEnemyIntoTeamAndKill",
  "kTurretsDestroyedBeforePlatesFall",
  "landSkillShotsEarlyGame",
  "laneMinionsFirst10Minutes",
  "laningPhaseGoldExpAdvantage",
  "legendaryCount",
  "lostAnInhibitor",
  "maxCsAdvantageOnLaneOpponent",
  "maxKillDeficit",
  "maxLevelLeadLaneOpponent",
  "mejaisFullStackInTime",
  "moreEnemyJungleThanOpponent",
  "mostWardsDestroyedOneSweeper",
  "multiKillOneSpell",
  "multikills",
  "multikillsAfterAggressiveFlash",
  "multiTurretRiftHeraldCount",
  "mythicItemUsed",
  "outerTurretExecutesBefore10Minutes",
  "outnumberedKills",
  "outnumberedNexusKill",
  "perfectDragonSoulsTaken",
  "perfectGame",
  "pickKillWithAlly",
  "poroExplosions",
  "quickCleanse",
  "quickFirstTurret",
  "quickSoloKills",
  "riftHeraldTakedowns",
  "saveAllyFromDeath",
  "scuttleCrabKills",
  "shortestTimeToAceFromFirstTakedown",
  "skillshotsDodged",
  "skillshotsHit",
  "snowballsHit",
  "soloBaronKills",
  "soloKills",
  "soloTurretsLategame",
  "stealthWardsPlaced",
  "survivedSingleDigitHpCount",
  "survivedThreeImmobilizesInFight",
  "takedownOnFirstTurret",
  "takedowns",
  "takedownsAfterGainingLevelAdvantage",
  "takedownsBeforeJungleMinionSpawn",
  "takedownsFirst25Minutes",
  "takedownsInAlcove",
  "takedownsInEnemyFountain",
  "teamBaronKills",
  "teamDamagePercentage",
  "teamElderDragonKills",
  "teamRiftHeraldKills",
  "teleportTakedowns",
  "thirdInhibitorDestroyedTime",
  "threeWardsOneSweeperCount",
  "tookLargeDamageSurvived",
  "turretPlatesTaken",
  "turretsTakenWithRiftHerald",
  "turretTakedowns",
  "twentyMinionsIn3SecondsCount",
  "unseenRecalls",
  "visionScoreAdvantageLaneOpponent",
  "visionScorePerMinute",
  "wardsGuarded",
  "wardTakedowns",
  "wardTakedownsBefore20M"
]

Well I can imagine that riot is making changes to the challenges right now. I bet when they are finished, they will update their documentation. And maybe that is just wishful thinking