Anything that requires hard chewing like nuts, raw vegetables and tough meats.

The first I ever heard of this topic was from reading the book "The Evolution of the Human Head" (2011) by Daniel E. Lieberman. It's an academic book, and parts are not exactly light reading targeted for the general public. I had read it when it first came out, seemingly well before it because such of point of discussion.

The problem with this topic is, if you try to look anything up on line you can quickly find yourself in the "manosphere" with its associated toxicity.

Not OP, but I remember as a kid being told to eat crispbread (freely available in schools, I assume in most of Scandinavia) because it was good for your teeth/jaw.

I guess one could also include chewy / starchy food; my Asian side family had similar saying but more towards chewing thing properly. (chew 100 times per food in mouth)

So things like crispbread, (raw) carrots, dried fruits/vegetables/meat/squid, etc

Does it go "crunch" in your mouth?

Even things like crackers may count, but generally hard foods include raw vegetables and certain fruit like apples and nuts.

Some people chew on the bones of meat.