Destiny 2 had it and people abused it by emulating an analog stick. Even though analog stick emulation will be worse than raw mouse, the auto aim was good enough to significantly overcome the drawbacks. Overwatch removed controller auto aim on PC specifically for this reason because it's basically impossible to stop on an open platform like PC.
On consoles some people abuse USB adapters that serve this purpose, but they look to be expensive and you'll still want a decent mouse to use it with. That functions as a bit of a barrier to entry.
When PC people already have a mouse anyway, downloading free controller profile software to spoof a fake controller is just too easy.
An ideal future solution in a world where everything was cross platform would simply be to match players based on input devices. If you want to use a mouse and keyboard on console or a controller on PC everyone still plays in a competitive and fair environment.
Hopefully they'll have a way to combat this and I believe destiny 2 had absurdly aggressive auto aim as well so maybe Halo won't be as bad of a situation if the same circumstances exist, but ultimately they really need to be prioritizing the main input device for the platform and I can't think of a way to truly stop people from using controller emulation short of a background application that aggressively reads processes/memory