Rather than putting a plate on top in the microwave, I have resorted to using a lightly dampened paper towel, gets the job done just as well and one less dish for me to clean.
Now I’m curious about the wet-strength resins in paper towels. Are they safe?
FWIW the USDA says plain paper towels are fine to wrap food while microwaving [1]. But I agree with the notion of wanting to minimize exposing food to toxins. I try to limit my microwave usage all together and prefer the oven for reheating food.