This was a wealthy Youtuber who offered $10,000 USD as a prize to the best video made to fit a hashtag. He gained millions of subs and earned millions of dollars in AdSense revenue from the contest but kept the money. When another creator called him out, he made a poor excuse for deciding not to pick a winner and said "I decided I may need that $10k someday in the future." I'd like to know if it is legal to offer a cash prize to generate 3rd party revenue if you never intend to deliver the prize. The Creator is Ricegum and the contest was called "The Clickbait Challenge."
No, it's not legal and it violates YouTube's own rules.
https://support.google.com/youtube/answer/1620498?hl=en
You can file a complaint with the California Attorney General. https://oag.ca.gov/contact/consumer-complaint-against-business-or-company