"Pork should be cooked."
Translation:돼지고기는 익혀야 돼요.
(1) subject/object markers are case markers. They are not strictly compulsory and can be dropped if the roles of the attached nouns or pronouns are clear from the context of the sentence.
돼지고기를 익혀야 돼요 = 돼지고기 익혀야 돼요 = You should get the pork cooked (thoroughly) => pork should be cooked (thoroughly)
(2) topic markers are auxiliary markers. They cannot be omitted as they themselves carry meanings ( just like conjunctions ), close to "Speaking of".
돼지고기는 익혀야 돼요 = Speaking of the pork, you should get it cooked (thoroughly) => Speaking of the pork,it should be cooked (thoroughly)
As their names suggest, these markers set up themes/categories for discussion (similar to the use of "#" in social media).
은/는 is not a subject marking particle the way 이/가 is. I think if we were going to mark 돼지고기 with 를 the sentence in English would be more like "You should cook pork". The English sentence Duo gives does not emphasize the action, but emphasizes pork as the topic of the sentence. We are talking about pork here and what it should be, not what someone should do to the pork
That's how I think of this sentence. It's one of the cases where you FEEL how 은/는, 이/가, and 을/를 are different.