I've never heard anybody say that keto causes brain fog. In fact a lot of people describes the opposite result which I would think most search results are about. There's also something called "keto flu" which some people experience for some time when they switch to a keto diet.

These terms are pretty vague and the best way to get a better understanding is too try keto yourself. It's just a different way if eating. You don't have to cook fancy meals and you can stop at any time.