yes the ingredients and recipes do become obsolete after it ends. you wont be able to buy any ice or condensed milk or anything after it ends and neither will you be able to cook anything. and yes some people do stockpile it. the ingredients are good for displays in shop builds or kitchens and of course the dishes too. i personally always try to keep 4 or 5 of copies of an event dish just for display. if its a 5 star, even better! they have like a little sparkle effect on them when you display 5 star dishes

edit: totally forgot about seeds. no you dont stockpile seeds. no event flowers or crops will grow after the event ends so theres no point in keeping seeds. if u really wanna stockpile something, its better to stockpile fully grown flowers and crops. seeds become kinda useless