Is aim assist allowed in runs? And if so, is it even helpful?

I ask because I've seen some people's PBs and it felt to me like part of the movement was... artificial? Like different from when I've seen some people playing live.

I looked around, and the only thing I could find was someone asking how to disable it. The rules don't mention anything. And one of the guide videos posted, you can see in the options that the dude has it on.

If it is legal, does it actually feel helpful? Like, would it help me get stuns on zombies/ivys?

It's allowed as far as the rules are concerned, but whether or not it's helpful or not is debatable.

Aim Assist is only allowed for NG+. Since most people are running assisted in that category anyway. However, most people use the Rocket Launcher so I mean it's kinda irrelevant even with that. During a speedrun, it hinders you cause there are some instances you want to shoot at a certain point on the body or whatnot. Aim assist sorta messes with that. So I'd say not allowed to use except NG+.