I get it that most of the animals generally called dinosaurs became extinct around 66 million years ago, and the Capsicum genus evolved only about 17 million years ago. So they didn’t coexist AFAIK.
But since birds, the modern descendants of dinosaurs, aren’t bothered by capsaicinoids in chili peppers, does that mean that dinosaurs could have eaten the peppers too?
So far as I know, it’s mammals specifically that are susceptible to capsaicin, not non-birds.
From a Nova special many years ago, I was given to understand that the dinosaurs lived long before the modern angiosperms (plats what make seeds) proliferated. Dinosaurs instead ate tree ferns and gymnosperms (plants what make naked seeds, represented best by conifers now.) Angiosperms make a great deal more protective chemicals, alkaloids and others than dinosaur digestion is prepared to handle.
Angiosperms are flowering plants, not plants that make seeds. (Gymnosperms are seed plants too.) They proliferated in the early Cretaceous, and by the late Cretaceous had become the dominant plant group, largely replacing the gymnosperms. So that while Jurassic dinosaurs ate mostly gymnosperms and ferns, ones in the Late Cretaceous had plenty of angiosperms to eat.
At least one scientist hypothesized that the development of protective chemicals by angiosperms might have led to the decline of the dinosaurs, but I don’t think this proposal ever gained much traction.
Since capsaicin receptors are specific to mammals, I don’t see any reason why dinosaurs would have had them. They almost certainly could have eaten chillies. You couldn’t use pepper spray to deter a T. rex.