This is an example of ellipsis, a common feature in informal English and news headlines.

It's also an example of pragmatics, specifically a conversational implicature, where the omission is forcing the reader to rely on the context of the sentence to fill in the blank or derive the meaning.

So while you are correct about the grammar, language use is often more complicated in practice.

As the other reply mentioned, another valid inference would be "it's not clear [that] YouTube can stop him." And as a general rule of thumb, you almost never actually need a grammatical "that"-- it's best to leave it out for brevity unless it's useful to distinguish the meaning of the sentence (typically when the parts before and after it are longer and the demarcation point could be interpreted several ways without it).

It's definitely valid as-is. It's debatable whether it's unclear (what would the alternative meaning be if you think it's unclear?)

Interesting question. This "feels" valid (as a native speaker) - the "that" or "if" is implicit - but not a rule I had ever identified before.

Looks like this may be called an "empty complementizer"; some more info here: https://en.wikipedia.org/wiki/Complementizer#Empty_complemen...