Chewing tobacco definitely causes cancer. There are clear correlations. They have to put warnings on the labels. Inhaled tobacco (“snuff”) causes cancer, too.
There are lots things you CAN chew and that many people do. East Asian “betel nut” chewuing (actually Areca Leaf chewing) also causes cancer. It’s frequently mixed with tobacco, which some people claim is the rreal cause, but there’s evidence that the areca causes cancer on its own.
Or you could chew coca leaves. Illegal here in the US. People have done it for thousands of years. I’m surprised to find people actually touting it as having health advantages. The cocaine content is low, and it’s claimed to provide vitamins and the low dosage of druf has supposed benefits. I’m skeptical, myself, but it bears looking into.
As for mint leaves, most essentrial oils are harmful in large doses, but I don’t think that’s an issue with chewing leaves. When I was in Boy Scouts we learned about finding and chewing leaves of teaberry and other plants.
One plant that we used to use, but is now not used is sassafras, sadly. We used to make sassafras tea from the root (and leaves, sometimes), but it’s been found to be carcinogenic. So i wouldn’t chew its leaves.