Good news for you, Ponyrama, MLPs are made from vinyl! I think mainly the plastics people are worried about weakening tend to be ones that are prone to being more brittle, or plastics that start out hard. Good rule of thumb with toys, if the plastic feels squishy it’s probably vinyl, if it feels more hard it’s likely to be something called ABS plastic. Think the texture of a pony vs that of a Dream Castle!
Another fun science fact that might ease your mind - hydrogen peroxide is also the active ingredient in Oxy Clean, which is widely recommended as safe for giving ponies a good deep clean, and, as far as I know, has been for quite some time.
I find it FUN how much science goes into toy restoration myself! Sometimes it feels very mad scientist. Only the creature on the slab is very cute and rainbow.