This construction is correct but rather old--fashioned and formal. We tend to use it more in formal contexts, such as essays and speeches. People would think it odd (I've deliberately used your construction here) ...if you were to speak in this way in casual conversation. Saying "I think her beautiful" would make you sound as if you were an actor who'd just stepped out of a Jane Austen costume drama. In everyday modern speech we would use two clauses, usually with the "that" omitted, and say the more standard phrasing: " I think she's beautiful".

The structure is a bit old-fashioned. We don't use it all that much at all nowadays. In the second area, relating to people, we use it even less. And would advise against using it. You might be better saying "I find her beautiful"