Legacy Lock controls in Fortnite are all about preference and whether you liked the increase in aim assist. Epic Games changed aim assist in 2019, but gave players the option to keep their preferred settings with the Legacy Lock.
In the advanced settings, players can choose Legacy Lock which will remove the new version of the aim assist and allow for the old one to take place. The one used since the game began. However, that's coming to an end.
Legacy Lock Controls Fortnite Explained
Starting March 13, Epic Games is removing the Use Legacy Look Controls because of unfair advantages. While the aim assist isn't as prominent as the current version, there is one distinct advantage. Players can spam the aim down sight button on their controller, and if you're near enough to an enemy, the crosshair will lock onto enemy players, even those behind walls.
Players were using this to cheat and find hidden enemy players or to lock on to them as you shoot a wall down, to instantly do damage to an enemy player. The only confusion on the issue is about timing. Players have been able to use this advantage for some time as PC players have complained about it for months, so it's a bit strange Epic Games decided to crackdown on it right now.
Fortnite players won't have to worry about the wall hacks for much longer.