But it's called a toast because that's what many UX people have called it for years, because it pops up and toast pops up. It's a specific kind of notification, not any notification. (There's also "butter bar" and "modal dialog" and so on.)

And yes it's behavior, but <input> and <button> and <select> and <textarea> define plenty of behavior too.

I think the big concern here is standards and process... not its name or concept in themselves.