Halo Infinite is the first Halo title in over a decade that allows players to use either a controller or mouse and keyboard, and its esport scene is no different.
Our aim is to be as inclusive as possible to grow the competitive community around the world. Because of that, we are going to allow players to compete on controller or mouse and keyboard regardless of platform. This means you might see entire teams on controller, entire teams on MKB, or teams with mixed MKB and controller players. We will also allow for MKB players to play on the Xbox Series X at live events if they’re in the open bracket. For those who didn’t compete in Halo because it wasn’t available on PC and was controller only – we welcome you with open arms and wish you the best of luck in the scene.
— "Designing The Halo Esports Ecosystem" (HCS Blog - July 2021)
The mouse and keyboard input is supported by the Halo Championship Series, Microsoft's official Halo esport league, and you can also play with a mouse and keyboard on Xbox consoles.
However, you wouldn't know that from watching any of the major Halo Infinite tournaments that have taken place so far. Currently, mouse and keyboard players are rare, you'll have a hard time finding even a single MKB player at an HCS LAN tournament with 500-1000 competitors.
The additional aim assistance that a controller provides has led every professional player to use a controller, even those who are talented on mouse and keyboard. Some players argue that the inputs will never be balanced and that the tracking required to hit shots during a 1v1 fight is simply too hard to do consistently on a mouse.
It remains to be seen if 343 Industries will buff the mouse and keyboard but given that the input is practically non-existent at the pro level a change seems likely.