Is it safe to bake brownies in an oven that had plastic burned in it?

ya it is­­­­­­­­

As long as all the plastic has been removed the oven is safe to use.

If you have not removed all the plastic, or can't you will notice an odor when you turn the oven on. If this is the only way to remove the plastic residue then leave the house until it is gone and take any birds with you and if you have a fish tank cover it as well as possible and turn off the filter.

Turn the oven exhaust on and open as many windows as possible.

You should remove the burnt plastic first. Plastic gives off nasty toxic fumes when burnt. It won't send you to the emergency ward immediately, but it is increasing your overall risk caused by exposure to toxins. Most homes are toxic enough already without burnt plastic fumes in your food.

No, you'll have to thoroughly clean the oven. Plastic fumes are toxic.

You should clean the plastic off first. It will affect the taste of the brownies.