You might try heating the plate first. Hopefully the plastic will become warm and melty enough to rub away with a cloth. Steel wool might also be an option, but you take the risk of scratching the enamel surface.
By mixing a paste of water and baking soda, and applying it to the area of the roaster oven where the bag melted, you should be able to clean the plastic off. If baking soda is not available you can try using cream of tartar.
To remove a melted plastic bag from the bottom of a car, let the car cool down a bit, but not completely. Take a metal knife and chip the plastic gobs off.
Ammonia is not a cleaner we'd choose to remove plastic from surfaces. It (ammonia) won't dissolve or "melt" the plastic, so it is ineffective in removing melted plastic from surfaces.
Most plastic bottles will be melted or burned by fire!
I believe in most cases it is just a good general idea to remove the melted plastic all together, just to be safe.
Melted plastic is generally non-toxic and usually a pain to remove. It is generally disposed of as trash, as it cannot really be re-used. If it's hot, it is hazardous as it can burn you and tends to stick to skin and materials. In that light, it is dangerous. Melted plastic is the material used in injection molding. That melted plastic is not "bad" as it is being used for a purpose (the manufacture of goods).
The plastic leg holders are oven safe and can be left in place during roasting.
WD 40 and a razor blade.
To remove melted plastic from a wood floor, you will need an iron and brown paper sacks or brown craft paper. Lay the craft paper over the plastic. Apply the hot iron to the paper and apply pressure just until the plastic becomes soft. Once the plastic is soft, peel it off the wood floor.
Melting plastic is tricky because you can easily char or burn it and can be dangerous because of the fumes released when the plastic melts. Also, different materials melt at different temperatures which can also make it difficult. If you have access to a pottery kiln, you could try putting the plastic in a kiln-safe container and melting it that way. However, it is unknown what will happen if you do so. Either way, I would not suggest melting plastic at home, just re-use it for something else or recycle it.
my advice would be to try using something more stiff to 'chisel ' it off
put it in a bowl of vinegar overnight and you should be able to scratch it off in the morning