I had a cast iron grate for my stovetop crack. I was considering using JB Weld high-temp epoxy to repair.
Says it is good up to 500F
The crack is on the edge, so not directly near a heat source.

Think this is an issue? Too much risk of toxicity?

If its something you cook on and eat the food, then I would definitely not do this. If it is only for aesthetics, then I probably would still not do it. I would braze it before using JB weld. That or find a replacement.

I imagine since you have cast iron that your stove is gas, and think you will exceed or get close to 500*.

I don't think this is a good idea. Epoxy generally isn't supposed to heat and cool many times. I don't think toxicity would be a major concern, but I don't think it would do you any favors.

Tried to use it to fix a hole in my smoker smoke stack. It sort of keeps the smoke going up and out. No way I'd use it on a cooking surface or to try to hold anything substantial.

Thanks for the info guys

I used it on wife's radiator once just to get it to Austin for trade-in before it overheated again. I would follow the other advice.