word usage - Why is it “Who do you help?,” not “Whom do you help?”?

I happened to watch a lecturer was explaining word order of English in the beginners’ English learning course in NHK’s - Japan’s largest and publicly-owned broadcasting network – educational TV program (aired on July 23rd). He showed four cubes, each of which showing the word, “Who”, “You” “Help” “Do” placed at random, and asked students to put the cubes in the right order:

Right answer: Who Do You Help?

I was comfortable with “Who do you help (speak / give / write, and so on) too, but a question arose:

Is “Whom do you help?” grammatically wrong or, obsolete? If so, why is it wrong, how and around when it became obsolete?

I’ve never seriously thought of such question as the declension of a dative pronoun in interrogative form until I hit upon the above TV scene. Taking advantage of this opportunity, I ventured to post a beginner’s question.