Riot Games just removed Killjoy from the agent pool in VALORANT after an exploit was discovered that let her place turrets under the map.
Killjoy was added to the game back in ACT 2. And since her release, she became a vital part of the competitive meta in Valorant. As she is a sentinel type agent, all of her kits are focused on defending a site from attackers.
Her kits like the turrets, nano swarm, and alarm bots are really effective in defending a site. Although after patch 1.11 sentinels agents like Killjoy received some heavy nerfs. Right now, she has to stay within range for her abilities to work. Even after all that, in competitive lobbies, her usage rate was rather high.
But a recent exploit forced Riot Games to disable Killjoy until any further notice. Apparently, a new map exploit lets her place her turrets under the map. As a result, some players are now abusing the exploits to get unfair advantages against their opponent.
Riot moved in fast and removed Killjoy from the agent pool. Therefore until the next patch arrives with a possible fix for the exploit, Killjoy might remain unplayable as of now. Riot has acknowledged the issue and is working to fix it soon.