Tap strafing is a controversial mechanic in Apex Legends that the developers decided to remove in order to preserve fair play in the game as much as possible.
Apex Legends is a highly competitive game where everyone wants to come out on top and that often brings in cheaters but also causes other players to actually improve their skills and use game mechanics to their maximum potential. Even the latter sometimes gets out of hand and the developers have to step in to keep the game enjoyable for everyone.
This was the case with "tap strafing" which is a repeatable manoeuvre that increases a character's mobility and offers an advantage, especially if there is cover around that can be entered at a steep angle. It involves mapping the forward movement and jump controls to the scroll of your mouse so they can be repeated extremely quickly.
Due to a few things under the hood of the engine Apex Legends is running on, whenever you move forward while jumping, the game will move your crosshair to the side a little. When this is repeated extremely quickly, it can provide mid-air 180 degree turns while preserving momentum so you can dodge fire or enter cover at incredible speeds that are not obtainable with regular inputs.
Respawn announced that the mechanic will be patched out in the future, disappointing many users of the technique, especially because it's not easy to master and they probably spent a lot of time practising it. On the flip side, without patching it out, controller users will always be at a disadvantage since tap strafing requires a mouse.
Overall, the change should be a positive one in general since the movement itself looks kind of wonky and the advantages it offers could lead to a radically different playing field in the future.